FxMixer, Mixer, FxMixerView: backported FX send support from master branch

There once have been huge efforts to implement FX send support in the
master branch. In order to make it available on a stable base here's
a backport which is non-trivial as there have been major rewrites of
the mixer's worker thread architecture.

There still seem to be bugs which we have to fix before merging into
stable branch.

Thanks to Andrew Kelley for the original work.
Merge branch 'stable-0.4' into stable-0.4-new-fx-mixer
FxMixer: rewrote loop for adjusting FX channel models in deleteChannel()
Song: added missing refresh of FxMixerView when loading project

Missed this call when backporting.
FxMixerView: fixed clear()

Before calling refreshDisplay() we have to actually clear the backend model.
Ported the FX mixer UI changes from stable-0.4
Merge pull request #25 from diizy/stable-0.4-new-fx-mixer

Ported the FX mixer UI changes from stable-0.4
Merge branch 'stable-0.4' into stable-0.4-new-fx-mixer

Conflicts:
	src/gui/FxMixerView.cpp
Merge branch 'stable-0.4' into stable-0.4-new-fx-mixer
Merge branch 'stable-0.4' into stable-0.4-new-fx-mixer
FxMixer: ignore mute state in addChannelLeaf() to fix lockup

Even if we do not process the FX chain later, we at least have to enqueue
all related FX channels to the job queue in order to prevent a lockup.
Merge branch 'stable-0.4' into stable-0.4-new-fx-mixer

Conflicts:
	src/core/FxMixer.cpp
	src/gui/FxMixerView.cpp
InstrumentTrack: set range of FX channel model to infinite when loading settings

When loading a project, instrument tracks usually are instantiated/loaded
before the FX mixer settings are loaded which results in an empty FX mixer
and thus 0 FX channels. The actual FX channel value for the instrument track
would be lost that way. Therefore set a big value which is being corrected
later by the FxMixerView for all instruments.
Added separate ZynAddSubFX repository as submodule
Initial integration of updated ZynAddSubFX
ZynAddSubFX: Use upstream icon file
VeSTige: adapt rename of MidiMaxKey constant
ZynAddSubFX: initialize IO engine after setting up synth instance
ZynAddSubFX: define PLUGINVERSION macro

Removes exit confirmation dialogs etc. in MasterUI.fl
RemoteZynAddSubFX: start/stop NIO system

Fixes crash when showing GUI of ZynAddSubFX.