Remove main.gd duplicates

[grandeisa]

Changes to core:

• Removes need for main.gd files
• Screen dimensions variable added to microgame.tres files that replaces WIDTH and HEIGHT constants found in main.gd files
• Removes need for register win/lose functions and respective signals by adding a function to Minigames autoload that checks if the node that called it is in the current microgame node or its descendant
• Removes empty main.gd files

All needed changes have been retroactively made to existing active microgames.