Summary:
Straight initial port of existing logic, with separate plugin metadata files
for different services. Perhaps could be changed to have just one normal
plugin, but with a config UI where the user can edit a list of
mediawiki-running servers and individual trigger words per server.
Left for another step/developer for now.

Test Plan:
A bug in Milou (kde #389611) prevents normal testing for now.
One can increase that reset timeout to some big value, e.g.
m_resetTimer.setInterval(3000);
in the Milou::SourcesModel constructor to work around the bug.
Then enable the mediawiki plugins you want to test, e.g. the
Wikipedia one, and enter in KRunner "wiki kde plasma"

Reviewers: broulik, jriddell, davidedmundson, sebas, mart

Reviewed By: mart

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10184