Optimizations, SVG export fixes, concurrent Roassal 3 fix

[JanBliznicenko]

Multiple fixes and improvements:

• increased efficiency of event handling
• hidden canvas grid when zoomed out too much to be drawn quickly (and too much to be seen well anyway)
• canvas grid now redraws properly when window is resized or camera zoomed out
• graph layout now works with diagrams containing self-edges (direct loops)
• removed some OpenPonk-related code (from classes that originally came from OpenPonk) and updated some comments mentioning OpenPonk
• multiple SVG and JS export fixes, including:
  • support for more shapes, especially line heads
  • if shape does not implement accept:, exception is thrown instead of infinite recursion
  • SVG now handles special characters properly (it does not need escaping of most characters like JS does - it breaks the SVG - same bug still in Roassal3)
  • SVG exporter supports line tails, not just heads - although that made the code a little too complicated :(
• Renamed asGroup to asRTGroup, because there were still issues with using asGroup in images where Roassal3 is present as well (it often created RSGroup where RTGroup should have been and caused errors). asGroup is still present, but deprecated with automatic rewrite.
• fixed errors when some RemoveCallbacks were removed by "chain reactions" while in a loop removing all callbacks (for example, element A depends on element B, we want to remove B, that triggers loop of all RemoveCallbacks of B, one of them is callback from element A that removes itself and therefore removes the RemoveCallback in B and that causes error, because B is still in the loop that triggers all these RemoveCallbacks, including the one now removed)

Most of the efficiency fixes come from @peteruhnak and help a lot in both Pharo 8 and 9, but it is still too slow in Pharo 9 because of the sheer amount of events generated by Pharo

[akevalion]

Thank you very much for your feedback