disabled form widget may have wrong hover status

[liucougar]

if a button disables itself when clicked, when it is reenabled by another button, the first button has hover status

Test case:

1. In test_Editor.html, try the undo/redo button in IE
2. make a change first, then the undo button is enabled
3. try to click it
4. now the redo button is enabled, and undo button is disabled
5. now click redo buton, redo button is disabled, and the undo button is enabled
6. but the undo button has hover status (the mouse now is over redo button, so it should not have hover status)

[liucougar]

(In [10222]) fixes #4200: disabled form widget may have wrong hover status