Follow use-header-bar settings in close confirmation dialog
window: add missing g_action_map_remove_action()
Don't leak the last GeditView on application shutdown

At first I thought all GeditView were leaked, so I searched where a ref
of GeditView is not released, and I've finally found.

But after some testing, only the last GeditView of the application is
leaked, because an unref in GIO must be executed in an idle, and the
idle is not executed on application shutdown.

So even if the leak was not really a leak, this commit will make
debugging memory leaks a bit nicer, and will hopefully save some time
for next developers.
Use GtkSourceEncoding in GeditEncoding
Move encoding utils functions to gedit-utils

gedit-encodings will be removed, but the utils functions will still be
needed in gedit.
encodings-dialog: use GtkSourceEncoding
encodings-combo-box: use GtkSourceEncoding
app: use GtkSourceEncoding
io-error-info-bar: use GtkSourceEncoding
commands: use GtkSourceEncoding
window: use GtkSourceEncoding
file-chooser-dialog: use GtkSourceEncoding
tab: use GtkSourceEncoding
Add gedit_document_get_file()

The function returns the associated GtkSourceFile.
Remove file loading and saving unit tests

The unit tests are in GtkSourceView.
GeditDocument: make load and save functions private

Make the following functions private:
gedit_document_load
gedit_document_load_stream
gedit_document_load_cancel
gedit_document_save
gedit_document_save_as

They are normally not needed by plugins. What plugins need is to do some
stuff before/after a file loading or saving, i.e. the following signals
are needed: "load", "loaded", "save", "saved".

By making the above functions private, it'll be easier to port
GeditDocument and GeditTab to GtkSourceFile.
Port to GtkSourceFileLoader
Port to GtkSourceFileSaver
Remove document-loader and document-saver

And their contained classes: document-input-stream and
document-output-stream.

All this code is now in GtkSourceView.
GeditDocument: remove unused code