Draft: patch the leak by clearing the lists on dialog close

Best fix I have so far for the memory leak #1322 (closed).

This helps somewhat on my system. Strangely it does much better when called artificially from a dummy code, to be investigated.

Maybe people suffering can test the artifacts from this MR.

TODO

• clear lists after ctrl-R on cancel or accept
  • split out just these Ctrl-R fixes to a separate MR?
• explicitly .clear() the pagescene before deleting it
  • fix crash spotted by @adrs0061
• the PageView (is that the right name?) in Marker might have similar leaks