Move drawing to one function

Moves to make it so that all drawing functions happen from one function. This will hopefully lead to an implementation of double buffering.
Fixed "delayed" clicks

Clicking on coordinates once for selection did not work with the previous commit. Now single clicks work.
Initial double buffering implementation

Added double buffering to the game window. Currently only updates the window on mouse clicks, but has no flashing during updates anymore.
GameWindow cleanup

Removed/fixed some comments and rearranged some code to be slightly more readable.
Fixed inital window draw

Window now displays correctly upon creation. Timer correctly updates the window every second.
Fix persistence of selection squares

Fix the initial draw code to happen only one time.
Reenabled antialiasing

Added antialiasing to all drawing. Also increased the border of the selection square, as it was hard to see with the antialiasing enabled.