STORM-192 FIXED Ctrl-Shift-W not to disable nearby chat toasts.

Bug reason: The fix of EXT-1419 disables showing nearby chat showing toasts at all whenever you close one.
That was done to prevent a crash when viewer is exiting, but closing a toast manually should be handled differently.

Fix: If a toast is being closed (destroyed) manually, just remove it from the toast pool
(to prevent further references to the invalid pointer), but keep on showing new toasts.
I've overriden LLFloater::onClose() to differentiate the two cases.