store imagemap in %pagestate, rather than a file

store imagemap in %pagestate, rather than a file

Using a file was sorta not right.

Note that when previewing, %pagestate is not saved, so
it has to rebuild the graph every time until that graph is saved;
then previews can use the cached data until the next time the graph
is changed.

Also note that it's stored in the destpage's pagestate. The imagemap
could vary between a page and an inlined page if wikilinks were supported.