Blender Git Loki

UI: File Close Dialog

This adds a new dialog that is shown whenever a file is closed.
So, either when a new file is opened, or when Blender quits.
The dialog allows to save unsaved changes. Furthermore it also
allows saving images that have been modified in Blender, but are
not saved yet.

Known limitations:
* Images that have no file path and have not been packed before,
are not saved.
* On MacOS the old dialog is shown when Blender quits.

Reviewers: brecht, billreynish

Differential Revision: https://developer.blender.org/D4860