Show (short) list of possible enum values when an invalid value is encountered (resolves #1318)

All enum errors have (hopefully) been changed to a luax_enumerror, which has a
fixed error message. If additionally a list of valid options is passed, it
lists that in the error message. For every enum error with few options I've
implemented this using a getConstants call.

This solution has been designed specifically to reduce the number of template
instantiations (as I've been told that was a concern). All new template code
happens in places where there already was an instantiation of the relevant
StringMap. Of course there is still std::vector<std::string>...

--HG--
branch : minor

src/modules/graphics/depthstencil.cpp

@@ -85,6 +85,11 @@ bool getConstant(StencilAction in, const char *&out)
 	return stencilActions.find(in, out);
 }
 
+std::vector<std::string> getConstants(StencilAction)
+{
+	return stencilActions.getNames();
+}
+
 bool getConstant(const char *in, CompareMode &out)
 {
 	return compareModes.find(in, out);
@@ -95,5 +100,10 @@ bool getConstant(CompareMode in, const char *&out)
 	return compareModes.find(in, out);
 }
 
+std::vector<std::string> getConstants(CompareMode)
+{
+	return compareModes.getNames();
+}
+
 } // graphics
 } // love