Add a "gicon" property so that a GIcon can be used instead of an
icon name, while still getting icon recoloring from the theme.
Also include a compatibility wrapper in libshell until GJS has
support for interface static methods.
https://bugzilla.gnome.org/show_bug.cgi?id=622451
Make StIcon compile and work in St.
Changes:
* ::icon-type and st_icon_set_icon_type are added to allow
specifying SYMBOLIC/FULLCOLOR for an icon.
* Ability to set the icon name from the theme is removed; it
wouldn't easily fit into our framework and two levels of
abstraction between code and image doesn't seem that useful.
* size CSS property is renamed from x-st-icon-size to icon-size
to correspond to what we are doing elsewhere.
* CSS and property based icon sizing are cleanly layered - if
you set the icon-size property, the CSS size is ignored.
* Add a simple JS test of StIcon.
https://bugzilla.gnome.org/show_bug.cgi?id=633865
