Constants for effect/volume commands

[MrEricSir]

Right now, users have to type in numbers to specify effect/volume commands. This isn't very user friendly.

We should have some type of system of constants in the C layer (#defines, probably) for each of these commands.

In Lua, etc. we'll need to find a way to either copy these or pass them through from the C layer.

[hryx]

These FX commands are #defines which look something like CMD_ARPEGGIO or VOLCMD_VIBRATO. With libopenmpt they live in a few places:

• the old libmodplug/sndfile.h
• soundlib/Snd_defs.h
• soundlib/modcommand.h

Not sure which of these is most pertinent to our purposes.

For Lua users, issuing such commands currently looks like:

song:effectCommand(channel, 17, 120) -- 17 means "tempo"

But should look something like:

song:effectCommand(channel, 'tempo', 120)

Modipulate-lua should map strings to these constants. This could be done easily by keeping a Lua table:

{
    arpeggio = 1, -- from CMD_ARPEGGIO
    portamentoup = 2, -- from CMD_PORTAMENTOUP
    portamentodown = 3, -- from CMD_PORTAMENTODOWN
    ....
}

Any value not in here would evaluate to nil, which Modipulate should ignore.