These icons are a relic of GLabel and were never implemented to
accomodate both image and text. This convenience can always be added
in the future, but no current instance assumes or needs it, so let's
replace them all with ImageWidget to show clearer intent.
Extra stuff done in this commit to facilitate the above (if you want to
really push my commit count, ask for more atomicisation):
- Register a bunch of widgets that are used in the process window.
- Allow setting the pid after the fact for the process state widget.
