Make classes not depend on the implicit GImGui context (#6199, #5856, #6199): ImGuiIO

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.

About ImGuiIO:
- ImGuiIO depends on ImGuiContext because some of its method want to event to `g.InputEventQueue`.
- To make ImGuiIO aware of the context to use, context which creates the ImGuiIO is given as argument of ImGuiIO constructor.
- The assert `IM_ASSERT(&g.IO == this && "Can only add events to current context.")` has been removed since it does not make sense anymore

NOTE: ImGuiIO could be completely independent of ImGuiContext if the InputEventQueue was moved from ImGuiContext to ImGuiIO, but since
ImGuiIO is a public class it would expose InputEvent type. Solving this problem is out of the current scope, but it is interesting to notice.

imgui.h

@@ -2022,6 +2022,8 @@ struct ImGuiIO
     // [Internal] Dear ImGui will maintain those fields. Forward compatibility not guaranteed!
     //------------------------------------------------------------------
 
+    ImGuiContext* Ctx;                              // Parent UI context (needs to be set explicitly by parent).
+
     // Main Input State
     // (this block used to be written by backend, since 1.87 it is best to NOT write to those directly, call the AddXXX functions above instead)
     // (reading from those variables is fair game, as they are extremely unlikely to be moving anywhere)