panel: Add workspace-switcher widget

[soreau]

Requires #340 for proper expected functionality.

[trigg]

I love the use of gtk overlay to make this, and the fact it's fully CSS themeable.

[AKArien]

tested quickly a bit, and i notice, if i switch the workspace row by anything else than the widget :

• if i changed workspace row, switching with the widget takes me back the previous row i was on last time i switched vertically by the widget (by scrolling)
• the workspace i am on will be at the center, and the widget’s representation of the surrounding workspaces will be positionned relative to the current workspace

[soreau]

tested quickly a bit, and i notice, if i switch the workspace row by anything else than the widget :

* if i changed workspace row, switching with the widget takes me back the previous row i was on last time i switched vertically by the widget (by scrolling)

* the workspace i am on will be at the center, and the widget’s representation of the surrounding workspaces will be positionned relative to the current workspace

Thanks, I'd noticed this too but forgot about it. Should be fixed now.