tab: remove timer's times_called field

It is used nowhere.
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
tab: use GtkSourceEncoding
commands: use GtkSourceEncoding
window: use GtkSourceEncoding
file-chooser-dialog: use GtkSourceEncoding
gedit_document_get_file(): returns a GtkSourceFile
Port GeditTab and GeditDocument to GtkSourceFile (not finished)