Save and load player inventory

[snipercup]

This requires #30 to work. I will publish this PR as a draft to make sure #30 gets merged first.

• When the player presses the travel button on the overmap, but before actually switching map, the inventory gets saved to General.player_inventory_dict and the dictionary gets saved to /current_save_folder + "/player_inventory.json"
• When a new map gets loaded, the inventory is loaded from General.player_inventory_dict
• When the player is at the main menu and presses 'load game', the player_inventory.json file will be read and saved in General.player_inventory_dict