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

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 the
sliders. This patch bases on assumption that sliders are always thicker
than progress bars, but guards against surprises by clamping from 0.

This also means that custom spacing of the parent ColumnLayout is no
longer needed, as it has only two children, and the equivalent Column
in BrightnessItem also has spacing set to zero.