feat: add support for parsing GroundOverlay with LatLonQuad bounds

[noberasco]

This PR adds support for parsing ground overlays with with bounds in LatLonQuad format.
In case bounds are provided in both LatLonQuad and LatLonBox formats, pre-existing behavior (get bounds from LatLonBox) is preferred.

Fixes #650 🦕

Example:

		<GroundOverlay>
			<name>Lipno_0_0</name>
			<Icon>
				<href>files/Lipno_0_0.png</href>
				<viewBoundScale>0.75</viewBoundScale>
			</Icon>
			<gx:LatLonQuad>
				<coordinates>
					14.034257,48.741688,0 14.051202,48.743284,0 14.049553,48.75094799999999,0 14.032606,48.74935199999999,0 
				</coordinates>
			</gx:LatLonQuad>
		</GroundOverlay>
		<GroundOverlay>
			<name>Lipno_0_1</name>
			<Icon>
				<href>files/Lipno_0_1.png</href>
				<viewBoundScale>0.75</viewBoundScale>
			</Icon>
			<gx:LatLonQuad>
				<coordinates>
					14.051205,48.743288,0 14.068161,48.74488199999999,0 14.066517,48.752545,0 14.049567,48.750952,0 
				</coordinates>
			</gx:LatLonQuad>
		</GroundOverlay>
... more ground overlays ...

become:

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