LibGUI+WindowServer: Implement drag-to-select behavior in GTextEditor.

To make this feel right, I needed to start passing keyboard modifiers along
with mouse events. That allows shift-clicking to extend the selection. :^)

WindowServer/WSAPITypes.h

@@ -109,6 +109,7 @@ struct WSAPI_ServerMessage {
             WSAPI_Point position;
             WSAPI_MouseButton button;
             unsigned buttons;
+            byte modifiers;
         } mouse;
         struct {
             char character;