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.

<- "Fusion" style

Current issues: No issues left.

1. The button permanently stays pressed down after a click for some reason. Fixed by switching from QPushButton to QToolButton. It still makes absolutely no sense to me though that the QPushButton stayed pressed down.
2. There seems to be a bug in the Breeze style that makes the bar show empty space next to it like it was expecting that text would need to be drawn there. Fixed with frameworks/kwidgetsaddons!224 (merged).
3. The margins look off when the space information is hidden. Fixed.