st-theme-node: Add support for -st-icon-style property

GTK+ added support for a -gtk-icon-style property in themes to
enforce a particular icon style. Do the same for shell themes
with an -st-icon-style property, with the same set of possible
values as the GTK+ variant:
  'requested' - use symbolic or fullcolor icon depending on the
                icon name (default)
  'regular'   - enforce fullcolor icons
  'symbolic'  - enforce symbolic icons

https://bugzilla.gnome.org/show_bug.cgi?id=740447