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/audio/Source.cpp

@@ -51,6 +51,11 @@ bool Source::getConstant(Type in, const char  *&out)
 	return types.find(in, out);
 }
 
+std::vector<std::string> Source::getConstants(Type)
+{
+	return types.getNames();
+}
+
 bool Source::getConstant(const char *in, Unit &out)
 {
 	return units.find(in, out);
@@ -61,6 +66,11 @@ bool Source::getConstant(Unit in, const char  *&out)
 	return units.find(in, out);
 }
 
+std::vector<std::string> Source::getConstants(Unit)
+{
+	return units.getNames();
+}
+
 StringMap<Source::Type, Source::TYPE_MAX_ENUM>::Entry Source::typeEntries[] =
 {
 	{"static", Source::TYPE_STATIC},