Also some other minor changes:
* fix transition when nagview is closed
* do not draw or update when not visible
* do not process events when not visible
* cleaned a bit the logic on next and show
* fixes#848
The NagView takes some actual space in the Y and when it appears
it cause the documents' content to be displaced.
The movement of the documents' content is annoying and should be
avoided so we draw the NagView entirely in overlay mode using defer
draw and we always keep its y size to zero to don't affect the
other application contents.
Jefferson González
Guldoman
Francesco Abbate
Takase