main: Add support for -high-contrast theme variants

While the default Shell style is fairly decent with regard to
accessibility requirements, having the ability to tweak certain
aspects where the regular style works less well is still useful.
For this purpose, try to load a -high-contrast theme variant of
the default stylesheet when a high-contrast theme is requested
(as determined by the GTK+ theme name).

https://bugzilla.gnome.org/show_bug.cgi?id=740447
st-texture-cache: Remove load_gicon_with_colors()

The split between st_texture_cache_load_gicon() and load_gicon_with_colors()
no longer makes any sense, so just move the code into the public method.

https://bugzilla.gnome.org/show_bug.cgi?id=740447
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
app: Use StIcon as icon_texture when loading from a GIcon

Themes - namely the HighContrast one - may now request symbolic
icons rather than fullcolor ones. In order to have recoloring
work as expected in that case, we will need a theme node to pick
up colors from - using an StIcon instead of manually loading a
texture from the cache gives us that for free, so do that.

https://bugzilla.gnome.org/show_bug.cgi?id=740447
app: Respect icon-style for faded icon texture

Just like regular application icons, the faded icon texture used
in the app menu should follow the theme's icon style setting.

https://bugzilla.gnome.org/show_bug.cgi?id=740447
theme: Add high-contrast variant

For now, simply enforce symbolic icons.

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