Windows, Internal: added experimental SkipRefresh mode. (#3515, #4763, #7556, #5116 , #4076, #2749, #2268)

currently: ImGui::SetNextWindowRefreshPolicy(ImGuiWindowRefreshFlags_TryToAvoidRefresh);
- This is NOT meant to replace frame-wide/app-wide idle mode.
- This is another tool: the idea that a given window could avoid refresh and reuse last frame contents.
- I think it needs to be backed by a careful and smart design overall (refresh policy, load balancing, making it easy and obvious to user).
- It's not there yet, this is currently a toy for experimenting.
My other issues with this:
- It appears to be very simple, but skipping most of Begin() logic will inevitably lead to tricky/confusing bugs. Let's see how it goes.
- I don't like very much that this opens a door to varying inconsistencies
- I don't like very much that it can lead us to situation where the lazy refresh gets disabled in bulk due to some reason (e.g. resizing a dock space) and we get sucked in the temptation to update for idle rather than update for dynamism.

imgui_demo.cpp

@@ -10,6 +10,11 @@
 // Read top of imgui.cpp and imgui.h for many details, documentation, comments, links.
 // Get the latest version at https://github.com/ocornut/imgui
 
+// How to easily locate code?
+// - Use the Item Picker to debug break in code by clicking any widgets: https://github.com/ocornut/imgui/wiki/Debug-Tools
+// - Browse an online version the demo with code linked to hovered widgets: https://pthom.github.io/imgui_manual_online/manual/imgui_manual.html
+// - Find a visible string and search for it in the code!
+
 //---------------------------------------------------
 // PLEASE DO NOT REMOVE THIS FILE FROM YOUR PROJECT!
 //---------------------------------------------------