GTableView and friends: API improvements for 'Sortable' flag.

Author: https://github.com/ignas-sa
Commit: 6228e18a09
Pull-request: https://github.com/SerenityOS/serenity/pull/442
Reviewed-by: https://github.com/awesomekling

Libraries/LibGUI/GModel.h

@@ -44,8 +44,9 @@ public:
     struct ColumnMetadata {
         int preferred_width { 0 };
         TextAlignment text_alignment { TextAlignment::CenterLeft };
-	bool sortable { true };
         const Font* font { nullptr };
+	enum class Sortable { False, True };
+	Sortable sortable { Sortable::True };
     };
 
     enum class Role {