Dragging of window, new hooks for derived classes, XP wnd access functions

[TwinFan]

Hi Chris,

thank you for sharing this code!

I had seen your forum thread on ImGui integration, checked out sparker's imgui4xp project and had added to it until it had all functionality that I think I need for turning LiveTraffic's windows into ImGui windows.

Here I want to offer you the changes to ImgWindow. I believe they are backward compatible for any existing code.

In a nutshell this is:

• Adding some public and protected function to allow working with the underlying XP window. That still doesn't allow changing the window handle, but allows using it.
• Added Move/Resize support for self-decorated windows (I use this for HUD-like transparent non-intrusive information windows)
• Added two more hooks before and after rendering, so derived classes can do stuff before (especially also add window flags) and afterwards
• Some cleanup (include dependencies, compiler warnings, compiling without ImGui deprecatons)

I have added individual commits to my xsb_public repository, separating the functionality in the same order as it originally had been added to imgui4xp, so you should have an easier time following which change supports which functionality.

Hope you find this useful

• Birger