UI Refresh

[svickars]

A big style update for the plugin UI. Opening the PR now, but happy to discuss any changes and will wait to merge until after merging #27.

• Move buttons to a sticky row at the bottom of the frame (will allow for standardizing with future plugins we develop)
• Add a menu button with links to GH readme and GH issues. Allows room for other features, if we want to monetize or ask for emails for access later on
• Clean up spacing throughout
• Clean up iconography
• Simplify button to generate variables node
• Redo output previews