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 remote-tracking branch 'upstream/stable-0.4-new-fx-mixer' into stable-0.4