Minor changes to level GUI

Now the win and lose screens show buttons to go back to the main menu,
instead of exit the game.
Main screen GUI changed

I know it is just a placeholder, but I couldn't stand the empty space!
now buttons are close together.
Sprites and camera reworked

Now sprites should be pixel perfect when rendered, as the size of the
orthographic camera changes depending on the resolution.

Also all sprites have been reimported as 64x64 pixels.
Misc Unity changes
Revert "Misc Unity changes"

This reverts commit 7260996045796cf705ba24a95ad4b571c4907ecb.
Misc Unity changes