Use GtkSourceEncoding

In the header, I've modified:
-typedef struct _GeditEncoding GeditEncoding;
+typedef GtkSourceEncoding GeditEncoding;

If it is an ABI break, another solution should be found.
Adapter between GtkSourceFile and file saving gedit API

The GeditDocument API for file saving and loading is used by some
plugins, especially the loaded and saved signals. It is simpler to no
break the gedit plugins as a first step.

This commit removes GeditMountOperationFactory (which was used only in
private functions) and uses GtkSourceMountOperationFactory instead. The
first argument to the callback function has been removed (a pointer to a
GeditDocument). The userdata argument is sufficient.
GEDIT_DOCUMENT_ERROR_CANT_CREATE_BACKUP is unused

So to simplify the code we can remove its support.
G_IO_ERROR_CANT_CREATE_BACKUP is used instead.
Convert error