Cinelerra + MIDI patch + DM2 Demo

All MIDI assignments are static at the time, and most of the functions
have only been superficially tested.

Basic technical description:

- I added a new class, MidiControl, in the same fashion as other
features are factored in Cinelerra.

- I hooked that class into the automake configuration and into the main
window startup code.

- The constructor opens an ALSA sequencer device, which has to be
connected by an outside app (jack patchbay, for instance).

- The class inherits Thread and is able to run a MIDI polling loop in
the background.

- Inside the MIDI event handling loop, there are switch-case-statements
which handle individual MIDI commands.

- The code called there is taken from various key handlers in cinelerra,
with minor parameterization changes.

My next step: understand how preferences are saved and provide a
preferences pane or a new config window for the MIDI section.

I'm having special difficulty in extracting the code for automation
control and for drag-editing, which would both be fantastic to have on
the MIDI control surface. Because the original code needs to handle
mouse press, drag, and release events separately, collecting the
corresponding code is a bit painful... So I'll defer that to a later stage.

The patch is relative to revision 1055, so everyone should be able to
apply it without trouble.