Imported more 2D art assets

- New splash screens (DOEGame\Assets\Images\Intro Images\New Placeholder
Intro Images)
- Pause Menu GUI (DOEGame\Assets\Images\Pause Menu GUI)
- Event Icons (DOEGame\Assets\Images\Event Icons)
- Start Menu GUI (DOEGame\Assets\Images\Start Menu GUI)
- Upgrade Icons (DOEGame\Assets\Images\Upgrade Icons)