Add LateInit() as optional plugin functionality

LateInit() is called if a plugin returns WANTS_LATE_INIT of its Init()
call.

LateInit() is called when all other initialization and setup has been
done, including first graphical update of the application. This enables a
plugin to place itself on top and gain focus if needed.

Signed-off-by: Per-Ola Robertsson <per-ola.robertsson@symbio.com>