Reset MessageList::seen_id in mail_reader_message_seen_cb()

Otherwise invalid source ID is used in the call of g_source_remove()
when is tried to remove "pending" auto-mark-as-seen timeout callback.
It can show a runtime warning on a console too.
Updated Russian translation
Updated Russian translation
Bug #727888 - Wrap the Location label in a reminder dialog
Bug #727967 - Undo/Redo text is not displayed in menus
Bug #684425 - Do not pass NULL text to gtk_entry_set_text()
Updated Slovenian translation
NEWS update for 3.12.1 release.
Post-release version bump.
Updated Indonesian translation
Updated Spanish translation
Updated Spanish translation
Newly created folders always marked as having subfolders

This corrects, a bit, a regression caused by commit b0106cc8, reported at
https://bugzilla.gnome.org/show_bug.cgi?id=722698#c13