#39 Selective fade/mute control node deletion

Clearing all keyframes in a track is excessive,
especially when there are effects with keyframes and
all I want to do is clear a few fade and mute control
nodes. (I know that keyframe icons can be slid and
merged into neighbors. Doesn't seem to be the case for
fade/mute control nodes.)

Some suggestions:
* Delete control nodes only in active region.
* Offer "Clear fade keyframes" and "Clear mute
keyframes" menu
options which operate only in the active region.
* Offer a "Clear keyframes" menu option in the effect
pop-up menu that operates in active region (and for all
of these, if no region is active, clear operates over
entire project).