Allow multiple cards to be selected in a card list,

Allow card actions to add/remove multiple cards.
not yet: actually use these multicard actions.

git-svn-id: svn://svn.code.sf.net/p/magicseteditor/code/trunk@853 0fc631ac-6414-0410-93d0-97cfa31319b6

src/gui/symbol/selection.cpp

@@ -25,7 +25,7 @@ void SymbolPartsSelection::clear() {
 }
 
 bool SymbolPartsSelection::select(const SymbolPartP& part, SelectMode mode) {
-	assert(part);
+	if (!part) return false;
 	// make sure part is not the decendent of a part that is already selected
 	if (mode != SELECT_OVERRIDE) {
 		FOR_EACH(s, selection) {