Moved responsibility for openining the Parts in a new Perspective from PerspectiveManager to PlaceManager

This makes the layering more consistent: PlaceManager was already responsible for closing all places
before a perspective change; now it is also responsible for opening those places.

This also eliminates the circular dependency between PerspectiveManager and PlaceManager.

This also fixes the Activity instance leak described in UF-138.
Integration test (a GWTTestCase this time) for regressions on UF-138.