Added support for the 'evdev' tablets

There are two complications:

1) In each event 'evdev' reports not all the axes preset, but
   only recently changed ones, so we need to keep the state of all
   the axes internally.

2) 'aiptek' tablet reports that it is a keyboard. Well, let us not
   disappoit it that it is not. Now we just check whether this nice
   "keyboard" supports xinput motion and proximity events.

BUG:332239,331572,329641
CCBUG:331917