Description of the new feature/enhancement

Currently block select is possible with alt+mouse. Since I use AltDrag to do window management, alt is not a valid key binding for me. On Linux this is ctrl+<mouse> by default. I'd like it to be an option to flip the keybinding.

Proposed technical implementation details (optional)

I dunno as keybindings don't seen like the right fit for this since mouse buttons is not valid in the keybinding syntax.