#1175 Toggle level visibility faster with middle-click on tab

[Pablo AB]

Right now to hide/show levels we can either right-click on the tab, then click on Make selected level (un)viewable, or Ctrl+Shift+H. Both ways only work for current level.

Would be very handy (and probably easy to implement) to toggle the visibility of any (not only current/selected) level by just middle-clicking on its tab/name.