Video subtitling for the GNOME desktop

This release covers some bugs that were quickly reported. Changes:

• Now builds with gtk-sharp 2.8 again
• Added the *.txt pattern to the "All Subtitle Files" filter, in the File Open dialog
• Print the stack trace to the console when an unknown exception is found while opening a file

Bug fixes:

• Application crashes at exit when MPlayer has already finished (#416895)

Thanks to:

• Marcin Zajaczkowski, for the feedback

Users having problems on File Open may want to run gnome-subtitles from a terminal to check the printed error messages.