Set default filename in Save dialogs

[humdingerb]

ArtPaint could prefill the Save dialogs' filename with the current image's name.

"Save project" dialog:

• If there's no project name, use the image's name (without the suffix).
• If there's no image name (i.e. you started from scratch), leave empty.

"Save image" dialog:

• Use the image's name.
• I there is no image name, use the project's name.
• If there's no project name either, leave empty.