Implement PlayerManager(), start RaceCountdown

-Uncomment PlayerManager() in loadInitialObjects()
-Implement PlayerManager::PlayerManager()
-Add some declarations for RaceCountdown
Code will NOT link
Add PlayerManager::raceCountdown_onCountdown

Code will NOT link
Add PlayerManager::onLevelUnload
Implement/Update LKernel:: getL/getG/get

Implement LKernel::getL and LKernel::get
LKernel::getG and LKernel::getL now return a nullptr on error instead of
throwing
LKernel::get throws if either getG or getL fail.
Start implementing networking classes
Add PlayerManager::onLevelLoad

Like the previous commits, code will still NOT link.
Typo (missing ; in NetworkEntity inline function)
Remove superflous private in KeyBindingManager
Declare UIGameManager and some methods

Still won't link.
Update Players/ classes

- Player's ctor now takes a pointer to LevelChangedEventArg, not a
value.
- Add virtuak key events to Player
- Create headers and sources for all the types of players
- Define HumanPlayer, implement its ctor.

Code does NOT build.
Define WiimotePlayer

And it's still not building correctly.
Define ComputerPlayer

Now fails at link-time.
Move LKernel get*() fncts back in header, add RVO

Add NRVO to the generic LKernel get*<>() functions since we use them
everywhere.
Fix obvious link errors by moving them back to the header.
Code WILL still NOT link.
Throw "Not implemented" in GameUIManager.

We need to link with GameUIManager::setItemLevel, called from
HumanPlayer's ctor, called from PlayerManager's onLevelLoad.
But this depends directly on the GUI system, and it's not even designed
yet.
And I'm not implementing the whole GUI system just because I said I was
implementing PlayerManager and its dependencies.
Have some unconditional exception throwing in the meantime.
Comment out PlayerManager in loadInitialObject

As a result of the previous commit, we'll comment out creation of the
PlayerManager singleton. Better than a runtime crash.

Code will still NOT link.
Implement PlayerManager::fillCharacterString

Code will NOT link
Throw "Not implemented" in WiimotePlayer ctor

We'd need Wiimotelib and I'm not implementing this right now either.
Code will still NOT link.
Finish resolving all the linking errors

- Implement all the dependencies of PlayerManager's ctor, except those
that depend directly on the GUI system or wiimotelib.
- Temporarily add OIS keymap, will be converted to SDL later.
Now compiles and runs correctly.