[2.0-m1] FileUploadField readonly does not affect button

[Colin Alworth]

When marking a FileUploadField readonly, the text field it uses for the path is marked readonly, but the button can still be clicked.

1. Create the field, and add it to the page:

   FileUploadField field = new FileUploadField();
   field.setReadOnly(true);
   formPanel.add(field);

2. Click the 'Browse...' button

Expected behavior: nothing should happen
Actual behavior: file window