Add symbolic icons to TextureCache's load_icon_name()

Add symbolic icons to TextureCache's load_icon_name()

Icons can be loaded as St.Icon.SYMBOLIC, FULLCOLOR, APPLICATION or
DOCUMENT. The first will look for a symbolic equivalent, the second
looks for a full-color version (and does fallback, eg, from
"drive-harddisk-usb" to "drive-harddisk"). APPLICATION and DOCUMENT do
full-color icons without fallback (as specified by the icon spec).

And update various callers to use the right flags.

Based on a patch from Matt Novenstern.

https://bugzilla.gnome.org/show_bug.cgi?id=621311