allow using extra buttons on disabled devices

[joukhaha]

Would someone else have use for the possibility to assign the default button differently for different devices?

As a workaround, I tested enabling the grabbing for the additional buttons of disabled devices, and made the last additional button of a disabled device behave as the default button. In this PR this feature is enabled if the -e flag is used to start easystroke, without -e it should be identical to the original.

The basic need behind this is to allow using button 1 for touch screens, while disabling button 1 for mouse, but still allowing default gestures with another mouse button. The added patch is included for archival purposes. The present version is obviously a hack which should be eventually reworked so that it integrates with the preference etc, if there's sufficient need for it.