Start implementing level loading, gui handlers

The code will compile but *NOT* link.

This creates files, declares functions and add boilerplate so that we
can implement directly loading a level from the main menu's "Single
Player" button.
Implement StaticGeometryManager::build()
Implement InstancedGeometryManager::build()
Declare MuffinImporter,MuffinImporte::parseByName
Implement Level::readMuffin()
Implement MuffinDefinition::getExtraFiles
Implement MuffinImporter::parseByName
Implement Level::readDotSceneAndSetupPhysics()
Implement Level::createEntities()
Implement MuffinDefinition::getThingBlocks()
Define Level::runLevelScript() to always throw

It's not implemented and throws "Not implemented" because it depends on
an unimplemented LuaMain function, and I have no idea how Lua works at
the moment.
Implement Level::runThingScripts()
Define loadLevelHandlers to always throw

Not implemented because too complicated at the moment. It relies on a
mess of RTTI that we don't need at the moment. The priority is getting a
bare level to load.
Fix missing namespace in LKernelHandler.cpp
Implement LevelManager::unloadLevel
Implement LKernel cleanup, remakeSceneManager
Update old Ponykart++ README : now using Ogre 1.9
Define unloadLevelHandlers to always throw
Implement LevelManager::invoke
Implement LevelManager::invokeLevelProgress