Containers: new Array class.

RAII wrapper around plain C array.
Added forward declaration headers.

Added it for all namespaces except PluginManager, as there is already
PluginManager.h. Will shuffle things around in later commit.
PluginManager: renamed *PluginManager to just *Manager.

No need to repeat the name, this was too long to write and thus
often wasn't used with fully qualified name. Also the name collided with
the namespace, which is unfortunate.
PluginManager: added forward declaration header.