oll-doc: Refactor HTML classes (continued)

Make a deeper class hierarchy
AbstractHtml
- AbstractOllHtml
  - OllDetailPage
    - HtmlDetailInline
    - HtmlDetailFile
oll-doc: Rename Snippets class to OLL

Start purging the "snippets" concept
oll-doc: Rename Snippet and SnippetFile classes
oll-doc: Rename SnippetDefinition and SnippetExample
oll-doc: Remove obsolete debug prints
oll-doc: Hide mainwindow.snippets as _oll
oll-doc: Further renaming of snippets to oll
oll-doc: Rename more snippets to items
oll-doc: Rename some owner to ollItem or oll
oll-doc: Rename last snippets to item

Now the word snippet is only left in comments.
oll-doc: rename snippets module to oll
oll-doc: Add header div in file output (back2ind)

Prepare a stub for a header div on top of the page.
Currently it only contains a plain text link to "index.html"
(which doesn't even exist yet)
oll-doc: Add index.html output

Create new class for that, move save() method to the abstract
base class, working on a filename attribute
oll-doc: fix issue in html template
Merge refactoring of HTML output (and index.html)
oll-doc: sort navigation lists by display title

Until now they were sorted by file (i.e. the core item) name.