Mouse control

- Middle- or Ctrl-click on a link should open it in a new tab (fg/bg according to `tabs.background`)
- When clicking the link with shift, `tabs.background` should be reversed accordingly.
- Ctrl + Mousewheel should zoom in/out
- Back/forward keys on mouse should navigate back/forward
- If `input.mouse.back_forward_buttons` is set to `false`, those buttons should not have any effect
- With `input.mouse.rocker_gestures` set, no context menu should be shown, but pressing left+right/right+left buttons should navigate back/forward
- When setting `input.mouse.rocker_gestures` dynamically, the context menu should be hidden/shown accordingly.