Add in placeholder levels

To be replaced.. just adding in so it doesn’t crash for now
Use Gdx.files.internal

Gdx.files.internal is the preferred method of getting at files
Use static accessor to get Game instance
Begin batch before drawing fonts

Fonts must be drawn to a begun batch.

From comments:
Anything taking a batch will just assume it has already been begun to
avoid calling begin()/end() unnecessarily.
Simplify GemGrid constructor

When the time comes that we have tens of properties, I don’t much fancy
having a constructor which is über long.

Instead, we can directly assign the properties after creating it.
Replace properties enum with String comparison

For now, you can just do String comparison on the string keys.
Otherwise, the game could crash if an invalid property is selected.

We could have a list of property classes somewhere like the enum, but
we can think about how best to organise that another time : )
Activate game once a level is selected

Once you select a level, we need to remove the LevelSelect from the
screen. I’m not fond on how I’ve done this — I just use clearChildren()
— but until LevelSelect is moved into TestGame.java, this will do.

Don’t disable the “create gems” button once it’s added.

Also, keep the level as an infini-level for now (selfish reasons: I
enjoy playing in that mode).