Move position of Alerts and Progress bar back to the center of the Statusbar.
Differential Revision: https://developer.blender.org/D9118
Reviewed by Brecht Van Lommel
Status Bar can show scene statistics, memory usage, version, etc set by context menu. Part two of T75672.
Differential Revision: https://developer.blender.org/D7557
Reviewed by Julian Eisel
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.
