foldermodel: lift screen add/remove handling out of screenmapper

screenmapper is the wrong place for it because we don't know the url
inside the mapper. Instead handle events in the foldermodel and
correctly delegate the add/remove actions to the mapper with both our
screenId AND the activityId

notably the previous code would incorrectly invoke addScreen with empty
activityIds triggering fallback handling that resulted in incorrect
m_screensPerPath management (there would be no screen associated with
our url anymore after a screenAdded signal because our screen got
associated with no url) and consequently firstAvailableScreen would
return no screen for our url (remember: we are associated with no url)
making FolderModel::filterAcceptsRow return false and filtering
completely valid entries

BUG: 467951

possibly also is the root cause behind
CCBUG: 467092