glib-compile-schemas: Improve some translatable strings

 • Fix capitalisation to be consistent
 • Use Unicode quotation marks where appropriate
 • Move trailing \n characters out of the translable strings and append
   them unconditionally

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=695573