Add is-waning to planet info map

It proved to be quite challenging to properly determine whether phase
angle is increasing or decreasing. So it's worth providing a flag for
this in the scripting API.

src/core/modules/Planet.cpp

@@ -1598,6 +1598,8 @@ QVariantMap Planet::getInfoMap(const StelCore *core) const
 		map.insert("phase-angle", phaseAngle);
 		map.insert("phase-angle-dms", StelUtils::radToDmsStr(phaseAngle));
 		map.insert("phase-angle-deg", StelUtils::radToDecDegStr(phaseAngle));
+		const bool waning = isWaning(observerHelioPos, core->getObserverHeliocentricEclipticVelocity());
+		map.insert("is-waning", waning);
 		double elongation = getElongation(observerHelioPos);
 		map.insert("elongation", elongation);
 		map.insert("elongation-dms", StelUtils::radToDmsStr(elongation));

src/scripting/StelMainScriptAPI.hpp

@@ -259,6 +259,7 @@ public slots:
 	//! - phase-angle : phase angle of object in radians (for Solar system objects only!)
 	//! - phase-angle-dms : phase angle of object in DMS (for Solar system objects only!)
 	//! - phase-angle-deg : phase angle of object in decimal degrees (for Solar system objects only!)
+	//! - is-waning : whether phase angle is increasing, signifying the waning phase (for Solar system objects only!)
 	//! - elongation : elongation of object in radians (for Solar system objects only!)
 	//! - elongation-dms : elongation of object in DMS (for Solar system objects only!)
 	//! - elongation-deg : elongation of object in decimal degrees (for Solar system objects only!)