app: add new accessor gimp_container_tree_view_get_name_cell()

(cherry picked from commit 78ad0327da4b85e5d6d2807b670f06abe47824fd)
app: in the quit dialog, mention if the image has been exported

Install a cell_data_func for the dirty image list's text renderer abd
construct the label manually, using the same string and logic as in
the close dialog.

(cherry picked from commit 9fe3da2bf242be83f2799f71281e27a4dc146baf)
app: make the "exported to foo" string in the quit dialog nicer

(cherry picked from commit 936780676bcf66c8fe89c511f36173bb43f270e4)
app: show file:// uris as filename in the close and quit dialogs

and make the quit dialog's "Exported to foo" string easier to
translate by constructing the markup in a separate step.

(cherry picked from commit beb184e3953bc77553c6fb0c723792ac919ba1a9)