Bounds checking for ImGUI tools

[Daltz333]

User should not be able to move popups or boxes outside the bounds of the window. When the window is resized, popups or dialog boxes should get migrated inside of the available space, or resizing should get locked beyond a minimum point.

[AlexDvorak]

Would WindowManager::DisplayWindows() be an appropriate place to implement this logic? . Presumably this would also require logic in the Window class?