Change the volume icon/mute button into a ToolButton

This adds a hover effect so the user knows it's a button.
Creates a reuseable SmallToolButton which overlays an IconItem on a ToolButton ignoring the svg's margins and ToolButton's padding.
Changes the spacing between the rows to 1px (0px causes the ColumnLayout to reflow and the label sometimes shifts).

Differential Revision: https://phabricator.kde.org/D5144