Change Reduce HP to Modify HP
Adds buttons and moves UI to allow user to modify HP in either direction by increments. Addresses sander1095#20.

Hello! Super cool to see you are still working on this! I will try to take a look at this in ~9 hours. If i don't have the time, it'll be Wednesday instead.
Thanks for your cool contribution!
I think it'd be good to do some more testing after you made it your changes. For example:
- If you modify HP with -30 when the character has 10 HP, they should go down
- f you modify HP with -30 when the character has 10 HP and max 10 HP, they should go down and die immediately because that is the rules of D&D 5e.
- Increasing HP only increases until current MAX.
- Modifying HP of a character whose turn it is currently not should not cause bugs or jump the player order around :)
Just mess about with the current features and try to not break any of them. I could have made this easier with unit tests but sadly I didn't write these years ago :)
@Malechus If I can help out with the PR or if it is overwhelming, let me know! We could even do this kind of stuff together if you'd like. Bottom line, I'd love to help you out! ;)
I appreciate all the detailed feedback here. Give me a little while, and I will go through these and push some updates.