Add OfficeIPCThread::WaitForReady.

For LibreOfficeKit we need to start up the event loop, and can't continue
initialisation until all the related (VCL etc.) setup is complete -- the
OfficeIPCThread is also enabled as one of the last items, and can be
used to indicate whether or not we can start actually working with VCL.

Change-Id: I0450b65584ddf6e8d02ce0c6e66e06f47841d1b7
Bin outdated / incorrect comment.

Change-Id: I7b8f97377b9606a2228f172f772d8051a64703d2
Use OfficeIPCThread::WaitForReady rather than sleeping.

This way we actually continue when we're ready to, rather than
dumbly hoping we wait for long enough.

This isn't entirely unproblematic though -- if we have no config
pre-prepared (i.e. first-run), then we just end up hanging on this
since soffice_main exits without doing anything to the OfficeIPCThread.
(Which is especially problematic for unit tests which specifically run
 on an empty config.)

Change-Id: I064fb500a224cfe37a0d3ba24b6154ffd72a71a3