trash: Rescan trash dirs before operations with files

Patches to use G_FILE_ATTRIBUTE_TRASH_ITEM_COUNT were recently proposed
for Nautilus and GTK+. We have to be sure that the item count is valid
and therefore we have to rescan the trash dirs before. Otherwise the
count is 0 if enumeration isn't called before, which is obviously wrong.
Similarly, trash dirs have to be rescanned before other operations
(i.e. read, delete, pull), otherwise the operations may fail with
G_IO_ERROR_NOT_FOUND if enumeration wasn't called before.

https://bugzilla.gnome.org/show_bug.cgi?id=711459