Removed the (now empty) panel manager modules.

Panel management is now extensible without the need to select alternative panel managers at
build time. The functionality of the old panel management modules has been merged and moved
into uberfire-workbench-client.
UF-84: documented that JAASAuthenticationService should not be used for web requests
Fixed startup problem for templated panels (was causing an NPE)
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.
Finished pushing down responsibility for panel/part definition management to the panel presenters

This caused a bit of a test refactoring, since there wasn't yet a good place to move the non-duplication
test that was previously in the PanelManager test.
Better error when trying to add a child panel to a parent that doesn't allow it
Fixed UF-142: splash screens don't come back when revisiting their perspective

This bug had multiple causes; all of them are fixed by this commit:

1. Splash screen activities (although their ApplicationScoped beans should never be
   destroyed) need to be shut down and restarted in order to display themselves.
   The PlaceManager was incorrectly set up not to tell the ActivityManager to shut
   down splash screen activities.
2. The SplashView was getting a new close handler added to it every time it was displayed.
   This was causing NullPointerExceptions on all but the first close attempt.
3. The AbstractSplashScreenActivity similarly was adding a new close handler every time
   the splash screen was shown. Same symptom as above.
4. Even with only one close handler installed, the cleanup() method was being called
   twice, thus firing two CloseEvents and causing an NPE on splash screen close