implement folding all folding also subregions

implements and close #394
Add option to copy (colored) document body only

Add an option that allows to export colored source code
- using CSS
- but only export the <pre>...</pre> element
- without a complete document

This makes sense in an environment where the CSS is already present as
an external style sheet (e.g. in the Lilypond blog ...)
Merge pull request #397 from uliska/copy-body-only

Add option to copy (colored) document body only
wording
update changelog
bump version to 2.0.16-dev
Merge pull request #398 from uliska/2-16-dev

bump version to 2.0.16-dev
Merge pull request #399 from PeterBjuhr/musicxml

Musicxml wip
add a method to get all the visible (or tabbed) panels at a certain dock side.

this is needed for new commands like "hide all left panels" etc.
multiple voices in pianoStaff wip