Initial rewrite of worldclock plasmoid and a new worldmap plasma "wallpaper"

Summary:
* installs a plasma-specific marble qml plugin in a private namespace
* exposes the property "radius" on MarbleQuickItem
* removes unused, but now conflicting Q_PLUGIN_METADATA macro from
  MarbleDeclarativePlugin (which is currently only part of a shared lib)
* some features of old worldclock plasmoid still TODO:
  * support for timezones
  * setting a lat/lon to center on

Reviewers: rahn, sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2354