gnu: mediastreamer2: Move the tester files and HTML doc to separate outputs.

The tester binary generated gets installed with about 20 MiB of data files,
and the HTML documentation weighs another 500 KiB or so.  Move them to
separate outputs.

* gnu/packages/patches/mediastreamer-srtp2.patch: Rename to...
* gnu/packages/patches/mediastreamer2-srtp2.patch: ...this, to please 'guix
lint'.
* gnu/local.mk (dist_patch_DATA): Adapt.
* gnu/packages/linphone.scm (mediastreamer2)[patches]: Likewise.
[outputs]: Add the "doc" and "tester" outputs.
[phases]: Add a phase to move the HTML doc and tester binary and data to the
'doc' and 'tester' outputs, respectively.
[description]: Remove the unicode characters surrounding the word 'powerful'.