evdev: Add implicit mode for touch tracking.

In implicit mode assignment of slot number and tracking id is performed
automatically on each synchronization requested by device driver.

This is done with creation of intermediate buffer for multitouch events.
This buffer holds untracked events until synchronization is requested by
device driver. It is needed as touch assigment requires
knowledges of all touch positions pushed in current and previous reports.

MFC after: 2 weeks