Make jp(e)g default extensions context-aware

This commit contains changes to Qt that enable two things:

• non-native file dialogs give .jpg or .jpeg extension depending

on the context:

1. If the current file is *.jp(e)g, suggest that extension.
2. Whether there are already files with this name

and extension in the directory, take that extension.

3. Take .jpg.
4. "All" filter now suggests .kra extension instead of .exr.

CCBUG:412651