Interactive character sheet

[tyoung-99]

Complete #9 first. Add features allowing user interaction w/ buttons/functions, instead of manually changing values.

• [ ] Spellcasting
  • [ ] Add round counter to active concentration
  • [ ] Add button to cast spell of each level (or 1 to cast spell, pick level?)
• [ ] Short/long rest buttons
• [ ] Add support for counters in abilities
• [ ] Add overall dice roller, as well as automatic dice rolling when clicking any modifier
• [ ] Make sure all counters have ability to increase/decrease, not just decrease