Add optional labels for Battery and Volume widgets

What this does
=====
This adds an optional ``label`` param to the Battery and Volume widgets. The label text is displayed to the left of the percentage readout.

Reason?
=====
Using a textbox next to one of these results in the padding causing a big space between the textbox and the percentage, and specifying a ``padding=1`` or ``padding=0`` affects the right-side padding of the widget as well.