This interface inly has a single implementation behind it, which is also
only used in 2 places after the previous commit. That's a lot of
additional complexity and compiler indirection for no good reason.
This change removes the interface and uses direct nsDocShell::Cast calls
instead of going through the interface in the few places left now that
we no longer build on a presentation context for links.
nsPluginInstanceOwner only listens to keypress events in the default event group.
However, in our changed operating mode, keypress events are not fired in the
default event group if the key does not result in something printable.
This means that nsPluginInstanceOwner should also listen to keypress events
in the system event group and should handle each keypress that way, but
only once. I.e., if a printable keypress event is received in the system event
group, it should be ignored, since it would've already been handled in the
default event group in that case.