Turn free space label into a flat button

Turn free space label into a flat button

The usage of a label to show this space information is problematic
because there are features/actions hidden behind clicking the
label. A button makes this clear. It also fixes the accessibility
of this control because previously this info would never be read
out by screen readers as it couldn't get keyboard focus.

A side-effect of this change is that the free disk space bar looks
more normal with non-Breeze styles. This had been a long-standing
issue.