Generate and install "locolor" icons.

Since we now have the capability to generate nice enough 16-colour
icons, let's install them into the "locolor" theme.

It also turns out that icon installation was broken by the recent
rework, so as a bonus the "hicolor" icons are installed again too.
In an attempt to avoid this happening in the future, add a new
installcheck procedure which does a very basic sanity check.