It's too easy to miss at the bottom of the screen, and if the status bar is
collapsed critical messages may not be seen at all. There might be a better
looking solution, but for now this solves the main usability problem.
* Add horizontal bar at bottom of all non-temp windows, similar to the Top-bar.
* Status-bar is hidden in UI-less fullscreen mode
* Current contents are preliminary and based on T54861:
** Left: Current file-path if needed. "(Modified)" note if file was changed.
** Center: Scene statistics (like in 2.7 Info Editor).
** Right: Progress-bars and reports
* Internally managed as own "STATUSBAR" editor-type (hidden in UI).
* Like with the Top-bar, Status-bar data and SDNA writing is disabled.
* Most changes in low-level screen/area code are to support layout bounds that differ from window bounds.
Design task: T54861
Main changes approved by @brecht.
