Added a simple dialogue screen in the beginning

I added a simple dialogue to add more "story" to the game.

Reorganized some files and tried to reduce the load on the main thread, however, I didn't succeed. I think I need to reorganize my way of equipping items on the player. Currently, weapons are being instantiated at the beginning and then a copy is added to the player tree, as a child of the weapon manager node. This proved much faster than directly instantiating and freeing on the fly. However, I think performance would greatly improve if the weapons were already on the player, and then simply "shown" when the weapon has been selected.