Draw meter into a template icon (not custom view)

If we render the meter in a custom view (as we were previously) we need
to know exactly which colors to use at a given time.  We had been
approximating this by using black in the normal state and white when
selected.  However this doesn't fully take into account all the states
that a status item can be in -- 10.10's Dark Mode is a good example
where those colors will be wrong.

Rather than try to render the colors ourselves, we can draw a monochrome
template image and set the status item to use that.  The OS will
translate that template image into the appropriate color.