app: tidy up image name display, separate export status (Fixes #3591)

This simplifies how image names/paths are displayed across the UI by no longer always stripping/decorating them, and moves getting an export status string out to its own function with corresponding title formatting flags.

Per the commit message:

Image display names & paths now include their file extensions and omit
any further decoration like brackets or export status, removing
gimp_image_format_display_uri() from gimpimage.c entirely.

An export status string can instead be gotten with a new public function
in gimpimage.c, gimp_image_get_export_status(), to replace always showing
it with an image's display name.

Accompanying flags for export status have been added to title/status bar
format strings, where %q displays the export status in full, and %Qx will
show a single character x if the image is imported/exported.