let engraver get args from the preview_mode widget

this way we can have multiple MainWindow and tool instances, having
different options. In custom engrave dialog the commandline now also updates
when options in the preview_mode tool are changed, we could even make
the window non-modal.
Merge pull request #261 from PeterBjuhr/master

Relative pitches supported
make Engrave (debug) a separate mode
add p&c and verbose checks to debugmode panel
make the custom engrave dialog simpler

the engraving mode can be chosen so there are no double settings anymore.
engrave (debug) button in debug mode panel
remove circular import dependency
add help button to debug mode panel
dont write datadir if it is False or probably None (in strange cases)

this hopefully avoids a TypeError message in rare cases on Windows
first stab at really simple markdownparser to make the userguide plain text and still translatable...
Merge branch 'master' of github.com:wbsoft/frescobaldi
wip simplemarkdown
barlines supported
basic clef support
wip simplemarkdown...
time tag attr symbol
rename Debug mode to Layout Control mode

no internal changes, also I think the -debug... commandline options are fine.
Its just the GUI.
Merge branch 'master' of https://github.com/wbsoft/frescobaldi