[applets/batterymonitor] Align battery progress bar margins with sliders & optimize it

Add vertical margins to the battery progress bar, so that the distance between colored bar and labels on top of it is the same as for slider. This patch bases on assumption that sliders are always thicker than progress bars, but guards against surprises by clamping from 0.

Please, read the rest in the commit messages carefully.

Before:

After (with extra margins):