Use embedded JSON metadata for binary plugins & custom for D-Bus plugins

This adds support for

• finding binary KRunner plugins in an own subdir (KPluginLoader paths + "/kf5/krunner") and using JSON metadata embedded in the plugins
• finding desktop-file based metadata about D-Bus plugins in an own subdir (QStandardPaths::GenericDataLocation + "/krunner/dbusplugins)

and makes this the new preferred plugin system.

Plugins and their metadata from the old KService-based system are still supported for backward-compatibility, but is marked as deprecated.

Related patches for porting runners to the new system:

• plasma/plasma-workspace!42 (merged)
• plasma/plasma-desktop!27 (merged)
• plasma/kdeplasma-addons!2 (merged)

See also patch for 3rd-party krunner plugins for how to support both old & new variant: kdevelop/kdevelop!139 (merged)