first stab at documenttree

dont use it bluntly as it parses al documents and is thus
very slow when e.g. switching sessions and opening many files at once
preview-mode help: comment about -ddebug-layout
preview-doc: Review "Engrave (Preview)" page
Improvements on the help text
simple module to help debugging frescobaldi
Message when musicxml2ly fails to create a ly-file. Issue #213
preview-mode: Rename configuration variables

Make them more consistent.
Noticed the need for it when writing documentation
preview-doc: Configuring Debug Modes

Add description how to configure Debug Modes through
configuration variables.

Now I think this is straightforward enough so we don't
need to clutter the GUI with yet one more Preferences page.
touchup debug.py, more can be done but it is already nice.
Merge pull request #214 from PeterBjuhr/master

Improvements on the musicxml import
Merge pull request #215 from uliska/preview-doc

Finish documenting Preview Mode
dont keep busy cursor while displaying error message
cant have empty translatable strings
simpler help doc text (remove text thats not necessary), -nd → --nd
no unneeded markup in translatable strings
nitpicks
pot/po/nl
oops