Add a framerate selector to save dialogs.

This is shown only when converting from a time-based file format to a
frame-based or vice versa and should be a helpful reminder that the
framerate needs to be set when doing such a conversion. Since the
removal of the video toolbar the only remaining framerate selector in
the main window is in the "View" menu, which is not obvious to affect
files written to disk.
"Main toolbar" shall now be "toolbar".
Use "I" as keybinding for inserting subtitles.

Remove keybindings from italic and dialogue actions.
Move "Select Video" item to the tools menu.

Hopefully this new location right below "Preview" will clarify that it
relates to the external preview and "Load Video" in another menu to the
internal player.
Remove status messages about files saved.

But keep them when saving as.
Use overlay backgrounds by default.
Add action to set the end time from video position.
Add wiki documentation on creating subtitles from scratch.
Update translations for 0.24.
Fix wiki page URLs to point to wiki.gnome.org.

Instead of live.gnome.org.