Basic UI support

[kanerogers]

Description

This is one of the last "big features" needed before we can ship 0.1.

UIs are big, scary things and I've tried as much as possible to avoid them with a 10 foot pole. But, games without UI are no games at all. VR games, doubly so.

TODO

• Refactor current functionality
• Add minimal working egui example
• Integrate egui into hotham
• Interactivity

[kanerogers]

Interactivity plan

• Use "aim" pose to get the start of a ray from the player's controller
• Cast a ray to see if it hits a Panel component
• Render some sort of ray indicator
• Render a "dot" on the panel that corresponds with where the ray has hit
• Have buttons highlight/toggle when cast
• Squeeze trigger == press button
• Haptic feedback on button hover
• Better button API