Working set fixes

This is a collection of commits that fix some issues with the working sets.

1. Create tests to check the basic behavior of working sets
   • Creating working sets
   • Deleting working sets
   • Switching between areas (i.e. from Code to Debug or Review) and maintaining the current working sets
   • Storing and restoring split view configurations
2. Fix bug 375446
   • Buttons of working sets active in another area were not displayed in the list of currently closed working sets.
3. Fix arrangements of working set buttons
   • The closed working sets widget was not resized according to its content, only after resizing the main window:
     
   • Closed working set buttons are ordered from left to right now. The most recently closed working set button is closest to the current working set button.
4. Fix bug 337891 and use the same configuration for a working set in all areas. It was confusing to find a different selection of files and arrangement of views in debug mode, than in the code area.
5. If a file is closed due to a switch of working sets, always ask whether it should be saved, even if it's still open in another area in the background.
6. Fix bug 409858
   • Do not try to open files from old working sets that are no longer existing.
7. Store the active working set in the session configuration, instead of the global KDevelop configuration
   • When starting a different session than last time, the old working set would not be opened, but only the welcome screen would show up.
8. Fix storing and restoring split view configurations
   • Store view ratios in working set configuration
   • Do not crash when restoring nested split configurations

Slightly unrelated

• Do not end debug session when temporarily switching from debug mode to review mode
• Cancel switching to review if saving the current files is cancelled