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.

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