Viewport: Fixed a viewport bug which led some popups to merge in underlying host viewport when they should be z-over their parent viewport.

Viewport: Fixed a bug where tooltips on their first frame didn't find a monitor leading to the "recovery" code to revert it to the main viewport for a frame. (#1542)

Viewport: Added support for minimized window which caused problem when ImGuiBackendFlags_HasMouseHoveredViewport was not supported. (#1542) + todo

Viewport: Added ImGuiConfigFlags_ViewportsDecoration to re-enable platform decoration (#1542)

Refactor: Moved README, ChangeLog, TODO files to docs/ folder + update Changelog. (#2036)