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