Make it possible to disbable notifications for certain highlights.

This is useful for "negative" highlights.
REVIEW:110261
Don't draw rounded edges on the OSD

It's borderline broken and looks ugly. As discussed in IRC, the OSD should
be refactored to use Plasma::Dialog in the future.
Revamp our decode pipeline.

What we used to do with incoming text:
- Determine whether there is any reason not to treat it as
  Utf-8, first by checking whether it meets the characteris-
  tics of certain popular Japanese encodings, then by check-
  ing it against a simple Utf-8 sanity checker.
- If it makes past those tests, just assume it's Utf-8 and
  decode it as such.
- If it doesn't, decode using the user's preferred setting.
- Unless that setting is Utf-8, then fall back to decoding
  as latin-1.

This was crazy stupid backwards (and likely motivated by
wanting to avoid Utf-8 decode costs as a fast path). Pro-
blems:

- Treating everything that looks like there's no reason it
  can't be Utf-8 blindly as Utf-8 doesn't work for all cases,
  and ignores user preferences.
- Because the Japanese encoding guesser and the Utf-8 sanity
  checker were coupled, it could actually reject some valid
  Utf-8 because it's equally valid Japanese - but then
  actually not do anything based on this info, because the
  user still needed to exlicitly select Japanese.

What we do now:
- By default, we decode using the user setting (which defaults
  to Utf-8).
- Unless the user setting is Utf-8 and the Utf-8 sanity checker
  tells us it can't be Utf-8.
- Then we run it through the Japanese encoding guesser, and run
  with its results.
- Finally falling back to latin-1 if we can't recognize a Japa-
  nese encoding.

This means there's no 'just read in Utf-8 as is' fast path any-
more, but then this was simply unreliable, and actually the new
implementation avoids some previous stupidity like reinstancia-
ting the Japanese encoding guesser for every line.

Meanwhile, the ability to cope with common mixed encoding channels
(i.e. the "German writing latin1 umlauts in an Utf-8 world" scena-
rio) is retained.

REVIEW:113823
Merge branch '1.5'