fix missing entity classes depending on FGD order (fixes #48)

an FGD with fewer classes loaded first could prevent entire groups of classes not being selectable in the next FGD. An empty FGD loaded first would prevent any class being selectable.

src/editor/Fgd.cpp

@@ -52,6 +52,7 @@ void Fgd::merge(Fgd* other) {
 		classMap[className] = newClass;
 	}
 
+	createEntGroups();
 	sortClasses();
 }
 
@@ -458,6 +459,9 @@ void Fgd::createEntGroups() {
 	set<string> addedPointGroups;
 	set<string> addedSolidGroups;
 
+	pointEntGroups.clear();
+	solidEntGroups.clear();
+
 	for (int i = 0; i < classes.size(); i++) {
 		if (classes[i]->classType == FGD_CLASS_BASE || classes[i]->name == "worldspawn")
 			continue;