Allow users to add spacers between articulations for bank list

[jtackaberry]

Add a spacer attribute to articulations that generates padding above the articulation.

Example:

//! c=legato i=legato g=2 o=cc:58,76
20 legato on
//! c=legato i=note-whole g=2 o=cc:58,81
19 legato off

//! spacer=1

//! c=long-light i=con-sord g=3 o=cc:58,86
7 con sordino
//! c=long-light i=note-whole g=3 o=cc:58,91
2 senza sordino

//! spacer=1

//! c=long i=note-whole o=cc:58,1
1 sustain
//! c=short i=spiccato o=cc:58,11
42 spiccato
[...]

The spacer attributes apply to the articulations below them, so those articulations' buttons should be inserted into the list with top padding.