Feature Request - Make ImGui and Viewport UI mutually exclusive

[AMZN-daimini]

Is your feature request related to a problem? Please describe. After discussing this PR: https://github.com/o3de/o3de/pull/11570 I am growing increasingly concerned with the interaction between ImGui and Viewport UI in the Editor. The PR mentioned above introduced logic to ensure that when ImGui is enabled, the Viewport UI moves out of view to allow users to click those menus. Similarly, ImGui is moved down a few pixels if component mode is enabled.

These adjustments are problematic since they require the two systems to know about each other's status (which isn't really necessary, especially since Viewport UI is an editor-only system while ImGui is also accessible in the Launcher) and also seem very brittle and easy to break when the interface changes on either side.

Describe the solution you'd like The simplest solution both in terms of code and UX would be to make the two systems mutually exclusive. When the user presses Home (or whatever button ends up being picked as a hotkey to enable ImGui) the Viewport UI should be hidden temporarily so the two interfaces don't overlap. This should include the Component Mode border. Similarly, when ImGui is disabled, the Viewport UI should come back.

Additional context Discussion may be necessary in case there are workflows that would benefit from both interfaces being visible at once. I am not aware of any, and most information on the Viewport UI would also be accessible via the rest of the Editor UI either way.