statistics: refactor QSG memory management

The code was wrong, because it deleted the ChartItems in the
main UI thread, not the render thread. This would delete the
QSG nodes in the UI thread and then crash on mobile.

Therefore refactor this part of the code by adding the
items to be deleted to a list that will be deleted by the
render thread.

As a drop in replacement of std::unique_ptr, implement
a silly ChartItemPtr class, which auto-initializes to null.

This turns the deterministic and easily controlled memory
management into a steaming pile of insanity. Obviously,
this can be made much more elegant, but this has to do for now.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

stats/statsgrid.cpp

@@ -20,7 +20,10 @@ void StatsGrid::updatePositions()
 
 	// We probably should be smarter and reuse existing lines.
 	// For now, this does it.
+	for (auto &line: lines)
+		view.deleteChartItem(line);
 	lines.clear();
+
 	if (xtics.empty() || ytics.empty())
 		return;