Add the target directory display name to the titles of some operations.

All operations that have only one target directory now have its display
name in the title of the corresponding ThunarProgressView, e.g.
'Copying files to "Test"...'.

This is accompanied by a new function called
thunar_file_cached_display_name() which takes a GFile and tries to
determine the display name of its cached ThunarFile, and otherwise falls
back to g_filename_display_name().