Ramp filter when rechaching frequency limits
Moved the diable fading to filteriir and cleaned up the filter effect code
used fading for the moog ladder as well
Merge remote-tracking branch 'upstream/master' into filter_ramping
Add effects preferences page.

Currently has a single tab that shows you the available effects. The tab
widget leaves room for future pages (e.g. quick effects, chain config,
etc.). The available effects page will be where you import and configure
external effects once we support external plugins.
Merge pull request #400 from daschuer/filter_ramping

Filter ramping
Add NULL pointer check.
Deere skin: Swap FILTER and KEY button order in mixer column. Fixes https://bugs.launchpad.net/bugs/1399549
Delete obsolete include directives
New SoundSource/AudioSource API

This is the first step towards a better/cleaner SoundSource API.
The operations for reading audio data have been moved into the
new abstract base class AudioSource.

Currently SoundSource inherits from AudioSource for technical reasons.
This is too much much coupling that needs to be broken up. I have
already some ideas on how to resolve it, but first this work needs to
be integrated.

During the refactoring I've noticed, that the quality and maturity of the
audio decoders for different file types greatly varies:

Good/mature:
- FLAC
- Ogg
- Opus
- WavPack
- libsndfile

Mediocre:
- MP3

Ugly/inefficient:
- M4A
- ModPlug

Untested AudioSource API:
- Apple CoreAudio
- Windows Media Foundation

Unfinished AudioSource API (just a hack):
- FFmpeg

Especially the code for the M4A decoder should be revised and cleaned up as
soon as possible, followed by a review of the MP3 decoder. Reading audio
data must work as stable and efficient as possible. This has priority over
the improvement of the internal software architecture!
Rewrite of the M4A decoder without legacy code
Slightly simplify the analysis of tracks