Make caching of the vocabs of sublessons work, now recursive view of lessons is fast...

Make caching of the vocabs of sublessons work, now recursive view of lessons is fast enough.
Still disabled by default as modeltest doesn't like me and asserts when attempting to insert an entry.
Start some feeble attempt to implement Cut for vocabs.

svn path=/trunk/KDE/kdeedu/libkdeedu/; revision=778458