Add class diagram for game

[chriskerrc]

I've added a draft class diagram for our game, drafted in draw.io. Here's the source file, via Google Drive: https://drive.google.com/file/d/1s5DDJUOR535FvkwZ5rYkuxzPcvK0oZah/view?usp=sharing

Could you have a look and see if any changes are needed?

• Are there any classes missing?
• There are some placeholder methods and attributes currently. What would you add or take away?
• Are the relationships between the classes correct e.g. 0..* (zero to many)?
• The white equilateral triangle head on arrows represents inheritance. Do we have any other types of inheritance?

I also created an images folder to keep the images in our repo tidy, and updated the name of the use case diagram image.

[chriskerrc]

I've made this PR a draft, to prevent early merging, because the class diagram isn't finished.

[chriskerrc]

To do (feedback from Alex):

• Venom should inherit from dynamic object
• Have a Consumables class, that the Food and Powerups classes inherit from
• Main menu inherits from Screens
• Add a Menu bar class that will be glued to the Map class

Next step:

• Make these changes
• Resolve conflict
• Merge PR