St: remove hover state if reactive disabled

The hover state of a widget can become persistent if
the widget becomes reactive while a pointer grab.
To avoid that, remove hover state if the reactive property
is disabled.

https://bugzilla.gnome.org/show_bug.cgi?id=728343
dateMenu: Don't make dateMenuButton reactive on current day

Having the ability to go to the current date if the user is already
on the current date can be confusing. So don't make the button reactive
until the selected date changes.

https://bugzilla.gnome.org/show_bug.cgi?id=726724