[RFC] weston: Sony clickpad support

[Alexander E. Patrakov]

This patch series adds support to weston for a special type of touchpads
found in some laptops. These touchpads contain one physical button that
covers the whole surface of the touchpad. Unlike the well-known Apple
touchpad, these touchpads have markings painted on them, that designate
virtual button areas. So the user interaction is quite different from one
gets from Apple touchpads.

+---------------------------+
|\/\/\/\/\/\/\/\/\/\/\/\/\/\|
|/\/\/\/\/\/\/\/\/\/\/\/\/\/|
|\/\/\/\/\/\/\/\/\/\/\/\/\/\|
|/\  Cursor-moving area  /\/|
|\/\/\/\/\/\/\/\/\/\/\/\/\/\|
|/\/\/\/\/\/\/\/\/\/\/\/\/\/|
|\/\/\/\/\/\/\/\/\/\/\/\/\/\|
+---------------------------+  <-- painted line
|   Left      |   Right     |
|  virtual    |  virtual    |
|  button     |  button     |
+-------------+-------------+
              ^
              |
        painted line

E.g., currently, in order to get a right-click on such "clickpad", one has
to click anywhere with two fingers. This is, however, inappropriate for
touchpads I am talking about. See: one needs to drag something, he places a
finger on the upper part of the touchpad surface (on the part of surface
designated for moving the cursor), then places another finger into the
left half of the button area, clicks with the intention to move the first
finger. However, what he currently gets is a right-click. Also, if one
wants to make a right click, he places his finger into the right half of
the button area, clicks, but currently gets a left click. In other words,
the touchpad is unusable for users that are accustomed to the way the
interaction with the touchpad is done in Windows.

I have implemented the interaction model that looks more like what one
gets in Windows. In order to do so, I had to refactor the existing code a
bit and add support for multitouch protocol to evdev-touchpad.c.

Tested all of this on my Sony VAIO VPC-Z23A4R laptop. Here is what works:

 * Moving the cursor by moving the finger in the cursor-moving area of
   the touchpad.
 * Not moving the cursor if one moves the finger in the virtual button
   area.
 * Tap-to-click.
 * Tap-and-drag gesture (unreliable).
 * Left and right virtual buttons.
 * Dragging, as explaining above.
 * Insensitivity to bad clicks (those outside the designated button area).
 * Two-finger scrolling.

...i.e. the touchpad is now quite usable. Zoom gestures are not implemented,
though.

I have also added autodetection of the interaction model based on the
laptops with clickpads that GUADEC participants brought with themselves.
Result: the current interaction model is valid only for Apple and Chromebook
Pixel laptops, for everyone else the alternative model in this patch is
needed because there are buttons painted on the clickpad.

-- 
Alexander E. Patrakov