Update boss behavior [WIP]

[weluvgoatz]

This PR contains probably one of the final big features for 0.7.0, which is the revamp of the two bosses, Yeti and Ghost Tree, to fit the standards set in place by the other aspects of the game. Here is a todo list:

• [x] Add pinch mode
• [x] Put the lives, display HUD and pinch mode activation behavior into a general "Boss" class that the Yeti and Ghost Tree inherit from
• [x] Revamp Yeti for code quality
• [x] Can't spam hit Yeti, it grabs and throws you away
• [x] Re-add yeti snowball throwing
• [ ] New sprites for Ghost Tree/Yeti

TO DISCUSS

• [ ] New behavior for Ghost Tree

Discussion for this PR can happen here, on Discord, or IRC, since the new actions of the bosses are up to a lot of interpretation right now.

Fixes #1083 Fixes #979