[lipstick] Handle surface pings in LipstickCompositorWindow

When a user tries to interact with an application that is not
currently responding to touch events, a visual indication
of unresponsiveness would be useful.

Added the following enablers for the visual indication:

 - Made LipstickCompositorWindow (LCW) handle the pings exclusively
    * since the surface is not exposing the serial outside,
      we need to keep only one ping in-flight at a time
 - Added a boolean property "unresponsive" to LCW
 - Added an invokable ping(timeout) to enabled pings from outside
 - LCW sends a ping on every Begin, End and Cancel touch event

A window is considered unresponsive if pong is not received
within the specified timeout. The compositor can then display
an indication after the window has remained unresponsive for a
period of time. After receiving the pong the unresponsive state
is cleared immediately.
what about caching this value? the fact that you have to reboot lipstick if you change it is an acceptable compromise, imho :)