UI Improvements

Closes #23 (closed), #33 (closed), #36 (closed)

Implemented features:

• Add an easy to use MessageBox
• Add fields for displaying the current balance of the player
• Implement balance check before building placement
• Implement closing of inspector and exiting build mode when esc is pressed
• Implement that attempting to select an already selected object deselects it
• Some UI layout and scaling fixes for different screen sizes and aspect ratios

Note: I'm creating this merge request because the planned features are implemented.
However, I don't think we should merge this until #38 is resolved.