Bug 878808 - [SMS] ensure the caret is visible when focusing the input area, r=gnarf37

Whenever the "Compose" area is focused or blurred:
 - the onscreen keyboard gets displayed or hidden, respectively;
 - the "Compose" area size changes;
 - its scroll position is reset to zero (scrollbar at top).

This patch adds a Compose.scrollToTarget() method to easily center a target (=
element or selection range) in the Compose area container.  This method is used:
 - when the Compose area gets/loses the focus -- see the scrollMessageContent()
      call in the ThreadUI.resizeHandler() method;
 - when an element is inserted into the Compose area -- see the scrollToTarget()
      calls in the Compose.append() method.