Improve repeater symbols

[adamrenklint]

Currently repeater symbols only work when placed directly after a note, making them useless when using blocking or non-blocking spacer symbols to define the note duration.

Change the repeater symbols to always look at the last real note, ignoring spacers:

/J__???;

J  1.1.01  dur144  vol100
J  1.2.49  dur144  vol 80
J  1.4.01  dur 96  vol 60

[adamrenklint]

This change is not as clear cut as I initially thought:

• should the repeated symbol be of the same length as the original?
• should the repeated symbol move the cursor the same as the original + spacers? i.e. J__?K becomes J__J__L