Generalise load stopper, sketch out future stopper

No functional changes yet
Move loading stopper view to header from first row

That way data does not get eaten... and it should be possible
to use the same logic with future stopper in footer.
Do not load future infinitely

Now this should work more or less reasonably
for smaller calendars too.
The agenda view needs to be scrolled before the
"load more" header and footer is displayed.
But at least infinitely looping loads should
not happen anymore.