externalvideo: Added support for various media sources

externalvideo: Added support for various media sources

Have added support for various media sources, which neccessitated the ability to support
iframes and page scraping for certain sources but not others. for this reason, I have broken
the sources out into their own classes, all implementing a common interface. This also
allows later additions to the sources to be added more easily.

Unfortunately language strings still need to be kept in the same place.

Other changes include the ability to set the width/height parameters as percentage values,
should the source need to be able to support that.

Change-Id: I802ffa8d8cf1172c9c45d1b71822d0d99c57b480
Signed-off-by: Brett Wilkins <brett@catalyst.net.nz>