FmXFormShell has a confusing mess of 3 of its own locks plus some
SolarMutexGuards in a couple functions.

But despite all that, most members are accessed without any locking.

First, add a "_Lock" suffix to all (non-API) functions that access
members or call into VCL, then add SolarMutexGuard in all API entry
points that call _Lock functions, and finally remove the existing
silly mutex members.

This should also fix the reported assertion (which probably doesn't
happen on master any more since the Scheduler has its own mutex now).

Change-Id: I215cdfe49ffe675229baa4b19ab419f440f0ffb0