Item-repository allocation/locking refactor

[Ivan Shapovalov]

Hi!

Following Milian's suggestion on https://git.reviewboard.kde.org/r/106945/, 
I'm putting the work on refactoring the "disk storage allocation" part of the 
item-repositories into a public repo instead of a review-request.

The refactoring is just about
- getting rid of locking in the item-repository directories (because we have 
rather good per-session locks);
- getting rid of numbered subdirectories inside of per-session cache 
directories (because nobody shall use and nobody actually uses multiple 
ItemRepositoryRegistries);
- changing the cache path once again
  from "~/.cache/kdevduchain/$session"
  to "~/.cache/kdevduchain/$appname-$session";
- making ItemRepositoryRegistry a true singleton;
- finally making the cache directories deleted when a session is deleted from 
disk.

The real reason behind all this was the last point.

The branch is called "itemrepository" and is on 
"git://github.com/intelfx/kdevplatform.git".

Thanks,
- Ivan