oll-doc: Clean up header field parsing

- remove the leading "\markup {" line from the field content
- return single element lists as a normal string
oll-doc: Add MetadataWidget()

A widget (may later be moved to a dialog or dockable panel)
to display (and later edit) snippet metadata.

This commit simply sets up the widget and places it in the
main window. On loading of a snippet it displays the content
of the snippet's metadata in line or text edits.
oll-doc: some cleaning up

- add code comments
- remove obsolete stuff
oll-doc: Add metadata.py to Eric project file
oll-doc: Update Eric4-TODOs
Merge branch 'doc-script' into reorganization