Merging file manager into main

See merge request !1

# Conflicts:
#   src/main/java/mi/hdm/TastyPages.java
#   src/main/java/mi/hdm/controllers/BaseController.java
#   src/main/java/mi/hdm/controllers/HeaderController.java
#   src/main/java/mi/hdm/controllers/MainPageController.java
#   src/main/java/mi/hdm/recipes/Recipe.java
#   src/main/java/mi/hdm/recipes/RecipeSearch.java
#   src/test/java/mi/hdm/recipes/RecipeSearchTest.java

Split GUI and the model into two classes. File manager is now fully implemented. the branch will be merged with main

Deserialize methods in FileManager now take the JSON string as parameter. Fixed bug where empty files would cause NullPointerExceptions. All classes now work depend on an instance of TastyPages instead of static class members in TastyPages

Categories, recipes and ingredients now saved in hashmap together with their associated code and a whole bunch of refactoring to go along with it. TypeAdapters for Recipes, NutritionTables and Categories. Saving and retrieving Recipe, NutritionTable and Category to / from JSON works (see tests).

simplified NutritionTable equals() method. created component classes for RecipeTile and CategoryCheckBox