pen button support

[wavewave]

Hi, all,

I just implemented two additional pen buttton support. One is for
lasso selection, and the other is for eraser. When you start pen
press, if one of the additional buttons are pressed, then it will
automatically change mode to lasso or eraser correspondingly. After
selection action (move or resize) is done, if you touch any other
unselected place, then it come back to original pen mode. You can
regard this as transient pen-to-select / pen-to-eraser mode switch.
This is a very useful feature when you taking notes while focusing on
something.

As you might notice, I uploaded several bug patches for version 0.6.4
to hackage. So if you are using version 0.6.4, please use the most
recent version. The new pen button function will be applied to the
next version 0.6.5.

Enjoy!

best,
Ian-Woo