GeditDocument: deprecate some functions

This commit deprecates:
gedit_document_get_location()
gedit_document_set_location()
gedit_document_get_encoding()
gedit_document_get_newline_type()
gedit_document_get_compression_type()

They have each a corresponding function in GtkSourceFile.
The code in GeditDocument is trivial, so it is not cumbersome to keep
those functions for a few development cycles. And probably
get_location() is used by lots of plugins.