Better screen removal handling

handleScreenRemoved never actually removed any view, as we were
comparing the screen id wit hthe screen order length, which is updated
only after the signal handler is done.
Also, make really sure to remove the desktop view from m_desktopViewForScreen
by searching the view itslef and not just by the index of lastscreen.
Crash in 465892 seems to indicate that sich a removal by index failed
leaving a dangling pointer in the map

BUG:465892