Add onBtnBuilder DSL to FXGL.kt

[AlmasB]

We currently have this:

FXGL/fxgl/src/main/kotlin/com/almasb/fxgl/dsl/FXGL.kt

Lines 718 to 729 in 43a1a24

	@JvmStatic fun onKeyBuilder(
	key: KeyCode,
	name: String) = KeyInputBuilder(getInput(), key, InputModifier.NONE, name)
	@JvmStatic @JvmOverloads fun onKeyBuilder(
	key: KeyCode,
	modifier: InputModifier = InputModifier.NONE) = KeyInputBuilder(getInput(), key, modifier)
	@JvmStatic fun onKeyBuilder(
	key: KeyCode,
	virtualButton: VirtualButton) = KeyInputBuilder(getInput(), key, virtualBtn = virtualButton)

A similar ButtonInputBuilder needs to be created with DSL functions similar to above.