Minor cosmetic fixes to search browser

Jonathan, here are my fixes to the latest help browser. I already tested these on my release branch, works like a charm. Since it's quite easy to browse the external documentation under extra now, I didn't bother backporting the code that I previously used to search the entire shebang. Works well as it is, and startup of the help browser is much faster that way. ;-)

I split this into separate changesets to make it easy for you to cherry-pick if you want. Here is a quick rundown of the changes I did:

• some cosmetic text changes
• null descriptions are shown as no description now, looks less scary :)
• empty search term shows the toc again, so we don't need another "home" button for that
• in addition to doc/, extra/ paths can be used to show a directory there
• beefed up the main toc a little; it now has entries for all the standard (vanilla) doc sections that seasoned Pd users will expect to find there, and I also added Cyclone and Gem at the end (might want to add some other external directories there, but I think that these are probably the ones that average Pd users will use most)

Since I mistakenly committed my changes to debuild (nw.js update) to my master branch already, these are included in this pull request as well. Sorry for the untidiness. Oh well. If this is a problem, I'll prepare a new pull request, just let me know.