Made amount attribute to DoubleProperty. Binded amount textfield in AddExpenseController to the expense amount property

Made handelAddBtn and handleEditBtn essentially be the same method, and differentitated them with enums

Reconfigured pom file for updated javafx version. Redesigned addExpense, Expenses and MainMenu, which is reflected in the fxml files

Implemented methods in AddExpense and Expenses such that information flow occurs between the two classes

Created new Expense Controller, renamed old Expense Controller to ExpensesController, and created two other new controller classes for testing

Created class for FileHandling GeneralBudget objects

See merge request !22

Budget

See merge request !11

Improved and simplified code in FileHandling class

See merge request !10