Tables: four small fixes.

Fixed last item flags leaking to disabled column, affecting IsItemHovered(). (#3651).
Validate and fix invalid DisplayOrder data from ini file.
Allow TableHeaderRows() to function will missing TableSetupColumn() calls.
Made TableHeader() use AllowItemOverlap mode to allow submit subsequent item in same cell, since it covers the whole cell area.

imgui.cpp

@@ -4749,6 +4749,7 @@ bool ImGui::IsItemEdited()
 }
 
 // Allow last item to be overlapped by a subsequent item. Both may be activated during the same frame before the later one takes priority.
+// FIXME: Although this is exposed, its interaction and ideal idiom with using ImGuiButtonFlags_AllowItemOverlap flag are extremely confusing, need rework.
 void ImGui::SetItemAllowOverlap()
 {
     ImGuiContext& g = *GImGui;