oll-doc: Add initial \markup parsing

Try to convert markup commands like \bold or the custom \ollCommand to
marked up HTML. This is necessary for the custom header fields that are
entered as \markup. With this LilyPondMarkup(object) class it is
possible to use markup commands that show correctly both in the compiled
LilyPond files and the HTML documentation.

Currently implemented:
- \ollCommand
- \italic
- \bold
- \typewriter

Items are correctly handled when the keyword is either followed by a
single token or by an expression surrounded by curly braces. Nesting
is not supported so far.
oll-doc: Add oll-usage as a header field

oll-usage is a markup field complementing oll-description.
The field's content will be displayed next to the description in the
generated HTML documentation, and it will be automatically displayed
in the titling of the LilyPond usage example.
git-commands: update oll-usage field

Move content from the usage example file to the definition's header.
oll-doc: Add sorted list of titles to OLL() object

The library now maintains a sorted list of item *titles*
beside the *names*.
git-commands: Change title

When sorting by item title it's better to have "Git" as the sort
condintion than "Print"
git-commands: Fix whitespace issues
oll-doc: Remove nav-bar from HTML file

Start of a reorganization of documentation output.
This navigation sidebar is not really necessary on the detail page.
It uses up a potentially large amount of space, and the TOC is better
placed on its own page - which is accessible through the link in the
header block
oll-doc: Restructure detail page layout

Now we have the output laid out in two columns
oll-doc: Add link list to other items in category

This list will optionally be displayed in the metadata section.
oll-doc: Clean up "Metadata" section in output
oll-doc: Add "include statement" to usage section
oll-doc: reflect changes in Eric4 project file
Merge branch 'doc-script' into reorganization