Add back the "blended tiles" property

The rendering will introduce blending optimizations at some point.
Fixed, effectively set blended tiles
Remove Gdx.files dependency by using a custom FileHandleResolver

Update javadocs accordingly.

Now only graphics resources impose the creation of a LwjglApplication: this
is probably subject to change in the future, where loading both maps
metadata and graphics resources should be made conditional.
Fix NPE due to checking Null parameters