glib::translate refactoring

[gkoz]

• Simplify ToGlibPtr. Remove ToArray and ToTmp. Use Stash for unified temporary storage. Don't bind temporary variables where the lifetime of the enclosing statement is sufficient i.e. almost everywhere.
• Make glib type an input type parameter instead of associated type for both FromGlibPtr and ToGlibPtr. Support both *const c_char and *mut c_char to avoid casting at the use sites.
• Fix the fn name in FromGlib.
• Tweak the DialogButtons implementation and related ffi definitions.

[GuillaumeGomez]

It took me while to read it, very big work ! Thanks !