Adds abstract File, Script, Stylesheet and Template model implementations for U4-933 U4-934 U4-935

Added a CssParser which is used internally by the Stylesheet class, but still needs a bit of work - primarily testing.
Resolves a couple of naming conflicts for Template.

src/Umbraco.Core/Models/DictionaryItem.cs

@@ -16,6 +16,8 @@ namespace Umbraco.Core.Models
         private Guid _parentId;
         private string _itemKey;
         private IEnumerable<DictionaryTranslation> _translations;
+        //NOTE Add this to LocalizationService or Repository
+        //private static Guid _topLevelParent = new Guid("41c7638d-f529-4bff-853e-59a0c2fb1bde");
 
         public DictionaryItem(Guid parentId, string itemKey)
         {