Allow custom articulations playback

There are a few problems with playback on the articulation markings now, for example:
1. staccato markings are much too short
2. stacatissimo markings don't play
3. marcato accents are too loud
et cetera, et cetera.

A good solution for all these problems, and one that would be adjustable to different styles and moods of music would be a menu that allows custom articulation definitions for tenutos/legatos, marcatos, staccatos, etc. throughout a piece.

I would imagine two sliders: the first, "length," would be expressed as a percentage of marked value. It would be defaulted at ~85% for normal notes, 60% for staccato, 95% for legato, etc.

The second, "volume" would be expressed as a percentage of the current dynamic level (ranging from about 50% to 200%). An accent mark should not have an absolute volume; it should be determined by context. Obviously an accent in pp is sounded differently from one in fff.