The new dash implementation is a single-column vertical sidebar,
whose items are scaled dynamically to fit the available height.
If the height is still exceeded after scaling down to a minimum
item size, excess items are cut off.
The now unused old dash implementation is renamed to OldDash, as
its code will be used as a base for the new view selector element.
https://bugzilla.gnome.org/show_bug.cgi?id=634948
