Make classes not depend on the implicit GImGui context (#5856, #6199): ImGuiWindow, ImGuiInputTextCallbackData, ImGuiListClipper, ImGuiStackSizes

This commit is a preparation toward adding ImGui apis with explicit context
and making ImGui applications being able to use multiple context at the same time
whatever their concurrency model.

This commit modifies ImGuiInputTextCallback, ImGuiListClipper and ImGuiStackSize so those classes do not to depend on GImGui context anymore.

About ImGuiInputTextCallback:
- ImGuiInputTextCallback depends on ImGuiContext because it has a
  `InsertChars` method adding character to `g.InputTextState`
- To make ImGuiInputTextCallback aware of which context to use, the
  appropriate context is given as argument of ImGuiInputTextCallback
  constructor.

About ImGuiListClipper:
- ImGuiListClipper apply to a context through its `Begin`, `End`, and `Step`
  method.
- To make ImGuiListClipper aware of which context to use, the
  appropriate context is given as argument of ImGuiListClipper
  constructor.
- Since the behavior is different than previously the class has been
  renamed ImGuiListClipperEx
- In order to preserve backward compatibility, a subclass of ImGuiListClipperEx
  named ImGuiListClipper has been defined and forward the implicit context
  to ImGuiListClipperEx parent.

About ImGuiTextFilter:
- ImGuiTextFilter depends on the implicit context because the Draw(..)
  method call ImGui::InputText(...)
- Instead from that commit the Draw(...) method takes an explicit context
  as first argument
- Since the behavior is different than previously the class has been
  renamed ImGuiTextFilterEx
- In order to preserve backward compatibility, a subclass of ImGuiTextFilterEx
  named ImGuiTextFilter has been defined. This subclass has a draw method
  override which and forward the implicit context to the parent class Draw(...)

About ImGuiStackSizes:
- ImGuiStackSizes was depending on ImGuiContext because of its
  `SetToCurrentState` and `CompareWithCurrentState` method
- ImGuiStackSizes is an helper object use
  for comparing state of context. It does not necessarily need to
  compare the same context. For that reason, as opposed to previous
  classes it takes the context it wants to compare to as argument of
  its method.
- For this occasion `SetToCurrentState` and `CompareWithCurrentState`
  have been renamed `SetToContextState` and `CompareWithContextState`
  to match the new method signature.

ImGuiListClipper

ImGuiInputTextCallbackData

imgui_widgets.cpp

@@ -3843,7 +3843,7 @@ void ImGuiInputTextCallbackData::InsertChars(int pos, const char* new_text, cons
             return;
 
         // Contrary to STB_TEXTEDIT_INSERTCHARS() this is working in the UTF8 buffer, hence the mildly similar code (until we remove the U16 buffer altogether!)
-        ImGuiContext& g = *GImGui;
+        ImGuiContext& g = *Ctx;
         ImGuiInputTextState* edit_state = &g.InputTextState;
         IM_ASSERT(edit_state->ID != 0 && g.ActiveId == edit_state->ID);
         IM_ASSERT(Buf == edit_state->TextA.Data);
@@ -3947,8 +3947,9 @@ static bool InputTextFilterCharacter(unsigned int* p_char, ImGuiInputTextFlags f
     // Custom callback filter
     if (flags & ImGuiInputTextFlags_CallbackCharFilter)
     {
+        ImGuiContext& g = *GImGui;
         ImGuiInputTextCallbackData callback_data;
-        memset(&callback_data, 0, sizeof(ImGuiInputTextCallbackData));
+        callback_data.Ctx = &g;
         callback_data.EventFlag = ImGuiInputTextFlags_CallbackCharFilter;
         callback_data.EventChar = (ImWchar)c;
         callback_data.Flags = flags;
@@ -4588,7 +4589,7 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                 if (event_flag)
                 {
                     ImGuiInputTextCallbackData callback_data;
-                    memset(&callback_data, 0, sizeof(ImGuiInputTextCallbackData));
+                    callback_data.Ctx = &g;
                     callback_data.EventFlag = event_flag;
                     callback_data.Flags = flags;
                     callback_data.UserData = callback_user_data;
@@ -4651,6 +4652,7 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
         if (is_resizable)
         {
             ImGuiInputTextCallbackData callback_data;
+            callback_data.Ctx = &g;
             callback_data.EventFlag = ImGuiInputTextFlags_CallbackResize;
             callback_data.Flags = flags;
             callback_data.Buf = buf;