Style scale value

[danirabbit]

Problem

We don't current ever show scale values, but in same cases it might be nice to show the value while dragging the slider

Proposal

Style scale value as an .osd element:

scale value {
background-color: alpha(@BLACK_700, 0.9);
border-radius: 3px;
box-shadow: 0 1px 3px rgba(0, 0, 0, 0.12), 0 1px 2px rgba(0, 0, 0, 0.24);
color: white;
padding: 3px 6px;
margin: -6px -6px -3px -6px;
transition: all 200ms ease-in-out;
}

scale:not(.dragging) value {
color: transparent;
background: transparent;
box-shadow: none;
}

Prior Art (Optional)

elementary/switchboard-plug-keyboard#447