Client submitbutton

Submitbutton is now disabled when there is nothing to submit.
Also show in title why the button is disabled.

The following settings can be used on the client:
- ```validate_on```: can be 'keyup' or 'change'
- ```remove_empty```: if TRUE, remove the empty option of a required select when a non-empty option is chosen
- ```disable_submit```: if TRUE, disable the submit button when there is nothing to submit or form is invalid
- ```use_disabled_class```: if FALSE, use the ```disabled``` class, this can give problems with some libraries
- ```submit_set_title```: if TRUE, set the button title to say why the button is disabled
- ```submit_title_text```: something like 'you cannot save because'
- ```submit_title_nochange``` something like 'there are no changes to save'