Fix attempt for X11 runloop problems

[jpcima]

This is a rewrite of the VST-X11 runloop system.
It's excepted to at least fix the current Reaper crashes, I don't know yet of possible regressions.

This is the big picture:
VSTGUI needs a runloop interface implemented, which allows it to register its event/timer handlers to the window system.
Somewhat conveniently, the VST3 host presents a matching interface, so one only needs to glue the 2 together.

To do the glue, each of the VSTGUI handler callback is wrapped into a VST3 object, that is then attached to VST3 by the interface provided.
One must keep track of all these wrapper objects, so we keep them in a vector.
Extra care is taken to stabilize the vector traversals, because additions and removals of handlers can happen inside a handler callback. The new strategy is to let the unused vector slots be zombie objects, with alive boolean as a marker, where indices never vary and the vector never shrinks, but the zombie slots get reused.

In addition:

• if a vstgui timer is removed when vstgui has no global RunLoop assigned (all CFrames deleted), the timer removal fails.
  Next time a runloop will run, the destroyed timer which has remained registered will crash it.
  The solution is to make sure to recreate/delete the editor on VST's open/close. GIven that we're now on the better update model, this must not be a problem.
• added a debug function to know what's in the runloop, in case it's needed to debug it again
• there will be error output if a handler is called without active runloop. this should never happen.
• there's disabled code in the RunLoop destructor, to eliminate the leftover handlers. it's not necessary if the host does its job, but it's left in case it might turn out useful.