demos/plplay: Keybinds to toggle UI and fullscreen

Currently for the demo plplay there is no way to fully hide the UI at runtime, so there is a new keybind added to toggle the UI. There is also a keybind to toggle fullscreen.

It's convenient to hide the UI when taking screenshots of the whole window or when trying to examine the output closely.

Keybids:

• H - Hide UI
• F - Fullscreen toggle