change output to handle any tree content, makes it easier to extend stuff

the parser also creates elements using a simple push and pop approach
some improvements
smarter
better
better method names

todo: make toplevel parsing line-based
forgot to save
make the block-level parser line-based

now code segments also can be indented and can occur in lists etc
and the parse_lines method accepts a generator to read the lines from.
of course, inline_code should just have a child element inline_text
oops
setup new manual in simple markdown format

this is still in flux, I need to
- implement translating paragraphs and list items and probably link texts
- automatic substitution of things like {key_help}, {author}, {menu_file_open}
- implement a way to mark pieces of md not to be translated, e.g. that
  {name:Wilbert Berendsen} is changed to {name} in the translatable string
  and that the value is inserted before display. (this is an example)

The end result will be that adding a help page is as simple as putting a
markdown file in the userguide/ directory, and that translatable strings
will be automatically extracted in a sensible way and added to the pot file
(just by creating a dumb .py file with all the messages).
disabled engraving after musicXML-import
engraving after musicXML-import deleted instead Issue #264
continue filling in and experimenting
Merge pull request #276 from PeterBjuhr/master

Disabled engraving after musicXML-import
wip new manual, not stable yet
cs update
readme for userguide writers
some more texts
upd README
all the layout mode help in the new format