Blender Git Loki

Fix: Fullscreen File Browser showing override warning on file open

Steps to reproduce were:
* Change File Browser display mode to fullscreen in Preferences
* File > Save As (make sure existing .blend is selected)
* File > Open
The file name would still be shown in red and the + and - icons would be
visible, which should only be the case for saving files, not opening.

Note that this change makes all `FileSelectParams.flag` values be reset
on re-opening a File Browser, which *may* in fact cause other issues.
It's easy to fix those though, and I'd prefer properly resetting the
flags and only keeping specific ones in that case.