Introduced tracking the leading finger of the drag gesture

Introduced tracking the leading finger of the drag gesture

This patch solves the problem of incorrect ending of tap & hold
gesture in Screen Reader mode and introduces leading finger change
during the gesture.

When Screen Reader is running and user performs tap+tap&hold gesture,
drag action can be performed. In current solution in case a user
touches the screen with a second finger during the course of the drag
gesture and releases the first finger before releasing the second
finger, the tap&hold gesture is not concluded properly - the drag
action is suspended without a proper closing with mouse up event.

This patch solves this issue and additionally provides possibility
of changing fingers many times within one drag action.

Change-Id: Ice65689d50883c9ef44a83695d04db974b4d7055
(cherry picked from commit c8989870a04b8c3cbd84f374c6fdbe2f5505c703)