Use a single global lock for the child frame state on Haiku

Use a single global lock for the child frame state on Haiku

Each BWindows (and various system callbacks) runs in a separate
thread, so there are very complicated locking semantics that
used to be handled manually.  This changes child frame state to
use a single global lock, which makes things much easier.

* src/haiku_support.cc (child_frame_lock): New variable.
(class EmacsWindow):
(~EmacsWindow):
(Unparent):
(ParentTo):
(MoveChild):
(FrameMoved):
(EmacsHide):
(EmacsShow):
(GetParentWidthHeight):
(OffsetChildRect):
(MakeFullscreen):
(SetUpDoubleBuffering): Use a single lock for controlling access
to child frame state.