Use case:
* When managing multiple terminals in a workspace, the borders makes it easier
to know where the focus is, but when there is only one it's obvious where the
focus is.
* When there's only a web browser for example, the borders are actually counter-
productive since it makes clicking a side scrollbar or a tab a bit harder (if I
smash my cursor to the side or the top of the workspace, I have to move it in
the other direction by just a few pixels to be able to grab it)
Behaviour:
* No borders when there's a single window in a workspace
* Borders when there are multiple windows in a workspace
fixes#2188
Michael Stapelberg