New Event Dialog: Fix input validation

[sevonj]

Issues:

The dialog was quite buggy.

• The Add Event button would be enabled even if the date was invalid.
• The Add Event button would not be disabled if the text box was cleared afterwards, unless the year box is touched.

Fixes:

Now the button will light up if and only if all input is valid.

• All fields are properly validated for the Add Event button to be enabled.
• All fields trigger rechecking of the Add Event button's status.

[lainsce]

Thanks for your help~