Scale: on mouse-induced zoom, make the zoomed window selected

If the zoomed window is not made selected the Ctrl+w shortcut will operate on
whatever window happened to be active, which will probably come as a surprise
to the user.
Scale: on mouse-click, make the clicked window selected

Not all mouse clicks lead to activation. One example is right-click, which
opens the window context menu. It then makes sense to make the clicked window
selected.
Merge pull request #2201 from autarkper/scale-zoom-select

Scale: on mouse-induced zoom, make the zoomed window selected