FormMain: fix menu accelerators breaking dynamic hooking

Fixes #133.

I haven't decided if this is a brilliant solution or a terrible hack,
but hey - it works.

As #133 described, PD's custom keyboard accelerators break dynamic
hooking, because the initial hook event (that catches the accelerator)
is locked mid-subroutine until the accelerator request finishes.  If the
event raised by the accelerator attempts to establish its own hook (such
as when a common dialog exits, and focus is returned to the main
window), it can't, because the accelerator hook has yet to exit.

To circumvent this, the accelerator control itself no longer handles
actual accelerator processing.  Instead, it activates a timer that runs
on a slight delay, then the accelerator hook immediately exits.

When the timer's delay has passed, it launches the requested
accelerator, and all hooking behavior works as it should because the
accelerator hook has already had time to wrap up its work.