prefs: add a language settings page.
prefs: start on GbEditorSettingsWidget

This widget will be used to manage the values of GbEditorSettings. When
selecting a language, this will be shown to configure that individual
language's settings.

It appears that this is going to be too big to do in a popover, so we
will probably use another dialog to conifgure them, as annoying as that
may be. At least it somewhat matches the design of Network settings
in the Control Center.
editor: add private child instances to template
prefs: wire up language list box to dialog with settings widget
prefs: use a dialog to show the language preferences.
ignore backup files ending in ~.
prefs: don't fill space in editor settings widget.
prefs: remove indent-on-tab settings
prefs: don't use deprecated properties
prefs: clear selection after closing settings dialog
editor-tab: remove indent-on-tab.
editor: hide current-line highlight during scroll_to_iter
snippets: don't use G_GNUC_CONST for get_type() functions.

According to the GCC documentation [1], const is stricter than pure and
should not be used with anything that modifies global memory.

[1] https://gcc.gnu.org/onlinedocs/gcc-3.1/gcc/Function-Attributes.html

https://bugzilla.gnome.org/show_bug.cgi?id=738557
settings: add schema for language settings in editor
prefs: ignore defaults from language list
settings: fix name of property
settings: add function to load language settings from gsettings
settings: load settings for language
editor: assign settings from language
settings: add bidirectional sync for configuration widget