Updated Turkish translation
window: Use view-context-menu-symbolic for the gear menu

This is a new, dedicated icon name for this purpose.
See https://wiki.gnome.org/Initiatives/GnomeGoals/GearIcons
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.
Port to 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
GeditDocument: make set_readonly() static
Port to GtkSourceCompressionType

And remove GeditDocumentCompressionType. This is an API break, but the
port is trivial.
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.