Style changes and fix warnings

[aaronfranke]

I like your demo, so I thought I'd contribute back to it a little bit.

• Conform to the GDScript style guide.

• Fix warnings (ignore unused signal warnings, rename shadowing variable).

• Use a named enum instead of an unnamed enum.

• Change add_timer to a private function and move it to the bottom.

[Federico-Ciuffardi]

Thanks!