textinput_v2: Make sendEnter/sendLeave the ones who decide the current surface

Instead of enabled/disabled. This made it possible for non-focussed
processes to interact with our virtual keyboard. In practice, this meant
that sometimes when switching applications, the disabled from the former
application would arrive after the enabled of the latter, leaving kwin
in a broken state (that the user could address by tapping on the screen
just once).