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

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, #340

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>