Activate gestures with modifier keys instead of mouse buttons

[Phanx]

Feature request:

Currently Gesturefy gestures only work with mouse buttons, which means I can't use them with a trackpad, since I can't "click" and move the cursor at the same time.

The Vivaldi browser, which has gestures built in, has an option to additionally activate gestures with cursor movement only (no click) while a modifier key (Alt, Ctrl, Shift, or any combination thereof) is pressed. This enables me to use gestures on my laptop even without an external mouse -- for example, with a mouse I can "press right button, move cursor up then down, release right button" to refresh the page, but I can also do "press Alt key, move cursor up then down, release Alt key" to do it on a trackpad.

I'd love to see this feature added to Gesturefy, if the Firefox WebExtensions API allows it.

Possibly related issues:

• #47 Keypress Gestures sounds similar, but explicitly limits itself to gestures on links.

• #98 Touchscreen support for gestures also sounds similar, but setting Gesturefy to activate with the "left mouse button" doesn't work for me (a comment on that issue notes that this no longer works in Firefox Quantum) and I actually want to activate with "no mouse button + a key press" (since I'm not on a touchscreen device and do have a keyboard).

System information:

• Gesturefy 1.2.1
• Firefox Developer Edition 60.0b6
• Linux Mint 18.3 Cinnamon