GeditDocument: remove comments explaining standard GObject code

And other small improvements to the header.
GeditDocument: remove useless #include's
GeditDocument: remove dead code
GeditDocument: save_metadata() static function

So dispose() is smaller.
GeditDocument: remove dispose_has_run attribute

doc->priv->location != NULL is sufficient.
GeditDocument: get_language_metadata() static function

To avoid code duplication.
GeditDocument: simplify code of guess_language()

Remove the 'content_type' paramater, it is an attribute of 'doc'.
GeditDocument: #define for a string used at several places
GeditDocument: inline set_content_type()

Inline set_content_type() into gedit_document_set_content_type(), and
use only the latter.
GeditDocument: small clean-up
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