Proj. explorer: Adjust order of load/unload signals

This tries to fix inconsistencies concerning the order of some signals
and the corresponding state of the sessions. In particular, it now emittes
aboutToLoadSession *after* aboutToUnloadSession. Also, retrieving the
active session name gives the correct result when updating project files.
In addition, there's a new parameter to aboutToLoadSession which describes
the session being loaded.

Change-Id: Iab5fb21080a7ee9bbf9f4d8958499cbc5549e81e
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>