Dynamic icons

[hylophile]

Hi!

This PR adds dynamically updating icons.

I also disabled caching for IconName as per https://dbus2.github.io/zbus/faq.html#why-arent-property-values-updating-for-my-service-that-doesnt-notify-changes, as otherwise some icons wouldn't update. Not sure if there's a better solution.

Little demo showing updates of nm-applet and mullvad-vpn:

eww.webm

[Nemo157]

This doesn't appear to work with syncthingtray. I've verified via dfeet that the IconPixmap changes when it connects/disconnects, but I'm not sure how to check whether the signal is being sent.

[hylophile]

@Nemo157 Just pushed a fix, syncthingtray should work now. (At least I was able to reproduce the problem and the latest commit fixed it.)

The fix is disabling caching for the IconPixmap property as well. Not sure what's different for mullvad-vpn, since it's also using IconPixmaps, but disabling zbus's cache seems to work.

[moetayuko]

this doesn't build after #3. created #5 to fix build