oll-doc: Update Eric4 TODO file
Add oll-base.ily file, to be included by all files

This file provides common functionality etc.
For now this is:

- initialization of all header fields
- markup commands for openlilylib documentation
oll-example: Improve usage example include file

define "includeName" as the path to be included
The module author only defines #(moduleName "...") with the
file's basename.

Add markup command for consistent documentation formatting
Provide initial \bookTitleMarkup for usage examples

Makes use of a number of header fields, including the
filename to be included.
git-commands: Add \gitIsCleanMarkup

markup command to return an arbitrary string depending on the
git status of the repository
git-commands: Update header fields
git-commands: Update Usage Example file

TODO: Make \gitDiff print on multiple pages.
oll-doc: remove mistakenly added file
oll-doc: minor syntax improvement
oll-doc: Cache HTML objs for display and file sep.

A Snippet() instance now has separate attributes for HTML
pages for display and file storage.
This increases memory usage but reduces HTML generation calls.
oll-doc: Factor out LibraryNavigation() class

This class generates HTML for a div with the navigation
structure of the library.