Disconnecting from WindowServer without explicit calls to DestroyWindow
would leave nulled-out WeakPtr's in the AppletManager applet list.
This led to a null dereference when adding a new applet, since we were
assuming the list contained no nulled-out applets.
This patch fixes the issue by always unregistering applet windows from
the AppletManager in ~ClientConnection(). We also do an extra pass of
pruning any nulled-out WeakPtrs from the applet list when adding to it.
Fixes#1551.
c56c8c8953