App menu: standardize Preferences/About/Quit

Add a separatoe between the preferences menu and the
about and quit menus. and replace "About Sound Recorder"
with "About".

See https://wiki.gnome.org/Design/HIG/ApplicationMenus
and https://bugzilla.gnome.org/show_bug.cgi?id=722868.

Signed-off-by: Meg Ford <meg387@gmail.com>