PointSymbol: Fix check for symmetry

[dl3sdo]

The test for symmetry in function isSymmetrical() was broken since the check for the position being at (0,0) was not negated.
Use std::all_of to improve readability.

[dl3sdo]

Basically, the minimum fix would be

return std::none_of(begin(elements), end(elements), [](auto& element) {
	return element.symbol->getType() != Symbol::Point
	       || !static_cast<const PointObject*>(element.object.get())->getCoord().isPositionEqualTo({0,0});
});

Instead of using none_of in combination with two negations that are ORed, I prefer std::all_of as this is closer to the description in the .h-file: "only point elements at (0,0) are allowed".

[dl3sdo]

The effect of the missing negation would only show up when importing an .ocd file where an unsymmetrical point object was rotated (I'm not aware that point symbols like that are really used):
OCAD:

Mapper (without fix) after import: