Moved everything Mana-related into the Mana namespace

Actually the ResourceManager, SpriteItem and also the MapItem and
TileLayerItem are all Mana-related. Moving them into the Mana
namespace avoids having to prefix excessively.
Dropped Mana namespace prefix where possible

The only places where it is still used is in main.cpp, in places where
disambiguation is needed and inside Q_PROPERTY macros.