MVCForm: upgrade support of form field placeholders

Now emptyText of model work as placeholder too as it was meant to.
I found out that in following comment in model Field class on line 410 for method emptyText:

    * if you are using this setting with a text field it will set a
    * placeholder HTML property.
Merge pull request #362 from DarkSide666/patch-1

MVCForm: upgrade support of form field placeholders