GeditDocument: remove all paramaters of the "load" and "save" signals

They are no longer useful. A plugin just wants to know when a file
loading or saving starts and ends. gedit takes care of the parameters.

The signal marshaller is set to NULL to use the generic marshaller.
GeditDocument: remove error param of the "loaded" and "saved" signals

Plugins don't need the error paramater, the signals are useful only on
success.

The signal marshallers are set to NULL to use the generic marshaller.
GeditDocument: remove unused code
Port to GtkSourceNewlineType

And remove GeditDocumentNewlineType.
GeditDocument: remove the "compression-type" property

The property has been migrated to GtkSourceFile.
GeditDocument: remove the "newline-type" property

The property has been migrated to GtkSourceFile.
GeditDocument: remove the "encoding" property

The property has been moved to GtkSourceFile.
GeditDocument: add a TODO comment
GeditDocument: remove the "location" property

The property has been moved to GtkSourceFile.
GeditDocument: remove GeditMountOperationFactory

It has been moved to GtkSourceFile.
GeditDocument: rename a function

get_language_metadata() is ambiguous. It is actually the string _for_
the metadata, not the language set in the metadata.
So get_language_string() is a better name.
GeditDocument: more documentation for the signals

Explain also the changes with gedit 3.14.
GeditDocument: small clean-up
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.
File browser: don't use deprecated GeditDocument API
Don't use the deprecated GeditDocument API