Optimize Shared-Pointer Usage

So far our API and codebase passes shared pointers by value. This should be made const& to optimize the superflous refcounting in such cases. This was also mentioned in one of the talks here: http://channel9.msdn.com/Events/GoingNative/GoingNative-2012 I (milian) will try to find out which one it was if anyone doubts the above statement.

Improve Session-Workflow

- beautify/redesign the "pick session" dialog ("kdevelop -ps")

Prevent .kdevduchain leakage

Some unit tests, maybe even temporary sessions in general, allocate a .kdevduchain session cache but never clean that up, leading to a huge mess there... It should also be tested whether this can happen if users create temporary sessions for debugging ("kdevelop -d ..."). And of course it must be asserted that the cache gets removed when a session gets removed.

Leverage Plasma for Welcome-Screen

Aleix worked on this in his GSOC, someone should pick that up and finish it properly.