Audit our settings model for thread safety issues

[lhecker]

It occurred to me today that we have a couple thread safety issues now. Since a single settings instance is shared across multiple windows (= threads), we must make sure that all getters are thread-safe. An example for such an issue:

terminal/src/cascadia/TerminalSettingsModel/Profile.cpp

Lines 384 to 392 in 52262b0

	winrt::hstring Profile::EvaluatedIcon()
	{
	// We cache the result here, so we don't search the path for the exe every time.
	if (!_evaluatedIcon.has_value())
	{
	_evaluatedIcon = _evaluateIcon();
	}
	return *_evaluatedIcon;
	}

It should be sufficient to simply find all getter methods that aren't declared as const in the header files. We should fix this not because it's likely to happen, but rather because it's an absolute pain in the ass to debug if it does (= heisenbugs).