Browse recursively for include from include path

This is something I was missing for a long time: I have by now a number of include paths specified in the preferences, but autocompetion of `\include` did not recursively search these directories, making autocompletion practically useless for that purpose. (For example I always had to look up when including a snippet from the openLilyLib snippets.

Now autocompletion
also suggest files from below the user-set include paths.

TODO:
- if the include paths contain identically named directories
  (I have several includes roots with a "layout" folder in it,
   other situations are quite plausible) then the next level will
   display a mixed (i.e. alphabetically sorted) list from all
   sources, which can be confusing).
   A suggestion is to group the autocompletion pop-up
   (something that would be good for other purposes too).
   There should be non-selectable group headers specifying
   - local files (relative to the document)
   - files from include paths
     - subheading for each include path
   - files from lilypond itself
   Group headings should only appear if there _are_ entries
   from it.