config: Add border gradients to windowrulev2 (#4335)

* Add border gradients to windowrulev2

* windowrule border gradient: Use CVarList to parse

* windowrule border gradient: No {} around short ifs

src/config/ConfigDataValues.hpp

@@ -16,6 +16,7 @@ class ICustomConfigValueData {
 
 class CGradientValueData : public ICustomConfigValueData {
   public:
+    CGradientValueData(){};
     CGradientValueData(CColor col) {
         m_vColors.push_back(col);
     };