File loading and saving

New public classes: GtkSourceEncoding, GtkSourceFile,
GtkSourceFileLoader and GtkSourceFileSaver.

https://bugzilla.gnome.org/show_bug.cgi?id=721016
Make sure text passed to insert_text is null terminated

Based on Jesse's commit d50108e57935871125cb42f275ebc2f096448452 in
gedit.

https://bugzilla.gnome.org/show_bug.cgi?id=726390
test-file-saver: print error when operation not supported