Accidental layout

#### Description

This pull request is a significant rewrite of the accidental layout engine, to bring it more in line with layouts as described by Elaine Gould (_Behind Bars_).  In particular:
- Not all accidentals need the default 3-line vertical clearance.  Double sharps require only 2.5 on either side; flats and double flats require only 2.5 below.
- Generally, when three notes have accidentals with vertical crashes, the accidental of the middle note is placed farthest to the left.  (This is not the case, however, when the interval of the lowest two accidentals is a second.)
- Tightens spacing for widely-spaced chords.

#### Examples:
Here's a comparison of the old layout engine and the new:
![vex-accidentals-revision](https://cloud.githubusercontent.com/assets/8905340/5248148/981f780c-7946-11e4-9034-d00bd7f7013c.png)