Remove needless fallback fullscreen/unfullscreen

They don't support dual display. We can always use
fullscreen/unfullscreen that are provided by GTK+. They will always
work well. :-)

Reported by uwabami. Thanks!!!
theme lightning-talk-toolkit: Support as-large-as-possible slide property

You can use the property to change font size per slide.

Force enable:

  = Large Title

  == properties

  : as_large_as_possible
     true

Force disable:

  = Large Title

  == properties

  : as_large_as_possible
     false