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.
- "All" filter now suggests .kra extension instead of .exr.

CCBUG:412651