Visual enhancements

Here's a bunch of additions and changes to make Party Hat more fun and appealing! They include:

• New icon + accompanying symbolic
• Illustration for the main view
• Widgetry changes to accomodate the illustration
• Increased default window size
• Metadata changes: updated the description and made the summary follow the HIG
• String changes to clean up the language a bit

The last two points include making some strings more "professional", but I think the added visual stuff allows us to do that while keeping the app fun and relaxed 😄

Previews!