[Editor] Enable creating Widgets in Editor

The Editor does not yet support XML syntax highlighting (the Java highlighter is used as temporary replacement), also the tab name of widget asset shows the actual file name (composed of UID) instead of the asset name provided by user.

proto/src/main/proto/project.proto

@@ -11,6 +11,7 @@ message Project {
   repeated ImageAsset images = 5;
   repeated EntitySetAsset entitySets = 6;
   repeated FontAsset fonts = 7;
+  repeated WidgetAsset widgets = 8;
 }
 
 message GameMapAsset {
@@ -45,4 +46,9 @@ message FontAsset {
   required string uid = 1;
   required string source = 2;
   required string name = 3;
+}
+
+message WidgetAsset {
+  required string uid = 1;
+  required string name = 3;
 }