Fix problem with backspace in open compositions

The backspace key needs special handling because it needs to close
the open composition but then shouldn't delete any additional
characters. Since it's not handled by IBus we need some extra
code. Previously the composition got cancelled but an additional
character to the left of the IP got deleted.

Change-Id: I81da74651b78e758916cc70b3a21f7c68886e7b0