[Core] Hierarchical menu for #1306.
[Core] PluginManager::GetFirstLevels().
[Core] Take subplugins into account for nested menus.

Closes #1306.
Merge branch 'master' of github.com:0xd34df00d/leechcraft
[Azoth] Support filtering Service Discovery, closes #1307.
[Blasq] ISupportsDelete::SupportsFeature().