Problems with new session support

[David Nolden]

I've changed the "Start New Session" behavior now, so that it doesn't close 
the currently active session.

That means that if you push "Start New Session", a new KDevelop instance is 
started in no time. It doesn't need to load any projects, neither any files, 
etc., so it's fast.

So now, if you want to open a session additionally, you just need to click 
"Start New Session", and then within the new window you 'switch' to the other 
session through the session menu.

I think a more complex management menu as proposed by Olivier as alternative 
to the dialog would be ok as well, but only as 'additional' structure, for 
example accessible through "Session -> Manage -> SessionName -> [ Open 
Additionally | Delete | Rename ]", so that it doesn't make simple switching 
slower.

Greetings, David