Remove the "Quickbar" feature and all things related to it

"Quickbar" was a feature of the Standard and CologneBlue skins that
allowed the sidebar to be displayed on left or right side of the page,
floated or fixed, and hidden on diff pages.

Standard was removed (Ia6d73c2d), and CologneBlue doesn't support this
anymore (bug 41246), so all things quickbar can now be safely removed.

* Removed user prefs option + interface
* Removed related messages
* Removed code for this in SkinLegacy
* Removed dead code in DifferenceEngine and Language

Change-Id: I5e6f7d48d6904a052a3a11547d3ebe6161463018