MDLSITE-2137 Skype icons filter rewritten.

- Now it's a proper 2.x filter.
- With standard version/lang/readme.
- Covered by unit test.
- Uses filter_phrases machinery, so it's cleverer.
- Uses the emoticon_manager to generate the img urls.

TODO:
- Convert some propetied to settings (fullmatch, casesensiteve).
- New setting to decide the formats where it's applied.
- With core changes, convert it to an iconset to be supported
  by the emoticon_manager properly (so they will appear in editor...).