touchpad: don't allow gestures with a clickpad button down by a finger

Note: this is just @mdmayfield's patch from !281 (closed) with a modified commit message and a test case added for that specific case. Closes !281 (closed) since it supersedes that one.

Allowing gestures when holding a physical click enables tasks like switching workspaces while dragging an icon, but this should only be possible with a thumb holding down the clickpad, not fingers. This commit restores the ability to hold down the clickpad with two or three fingers to right- or middle-drag.

Fixes #339 (closed), #340 (closed)