Tiled importer, more Gui stuff, test level

FEATURES:
- Changed the default resolution to 856x482
- Added another version of Arial, “Arial_Small” that doesn’t use
Distance-field rendering and thus looks better in smaller font sizes
and when downscaling
- GuiScreens now have access to the main class.  If you would like we
can also consider making the main class a singleton
- Made the main menu into an actual GUI with a play button
- Created a test level in Tiled using open source graphics from Kenney
- Created a level loader and an in-game GuiScreen and implemented the
test level with this.  As of right now it isn’t playable per se (it
just scrolls with the arrow keys) but it does render and loads
successfully from a .tmx (Tiled) file.