We might not get back the same GFile we are using for a save-as here via
gtk_souce_file_get_location(). So instead keep the GFile we created so
that our admin:// elevation has the appropriate URI.

Fixes: #730

If the file path has an ampersand, the path won't be
displayed. Set use-markup=False to fix that.

Closes https://gitlab.gnome.org/GNOME/gnome-text-editor/-/issues/732

Fixes: #732

If we do not have automatic settings, the compiled value will come from
the default in this struct. Set it to on to match our expected default.

Fixes: #733

Fixes: #741

This also ensures that the encoding used for the document is selected in
the save-as dialog.

Fixes: #743

In GtkSourceView, it must handle invisible sequences correctly. That can
have drastic performance issues on minified files which we sometimes see
in Text Editor.

Until something more concrete can be handled upstream in GTK and/or
GtkSourceView, mitigate the issue by assuming all text is visible as is
the case in Text Editor.

Fixes: #742

Fixes the remaining issues not caught by the previous fix.

Fixes: #741