dvb.media_addon: Trigger loading of media plugins.

- This is the workaround as described in the bug report #11018.
- I would have preferred to place this workaround in DVBMediaAddon::InitCheck()
  as this method can return a status code but unfortunately it is never called.
dvb.media_addon: Remove superflous init check.

- This init check prevented the MediaStreamDecoder from setting up an input
  format. The values the init check is based on are initialized by the first
  call to BMediaDecoder::SetTo() as described in the BeBook ("If you use the
  empty form of the constructor, you'll have to call SetTo() to establish the
  format to be decoded before calling Decode()"). But MediaStreamDecoder
  doesn't provide a public way to call BMediaDecoder::SetTo() first.
  There are at least two possible solutions to remedy this situation:

  1. Remove the init check
  2. Add a public way to call SetTo()

  The first solution was chosen for the following reason: Least amount of
  code change needed as it is an implementation detail so no client code has
  to be touched.
  The second solution might make sence when you want to stick to the BeBook
  where a clear distinction between SetTo() and SetInputFormat() is made, but
  as the current MediaStreamDecoder implementation doesn't stick to the BeBook
  anyway there is less need to add another method at this time.