Implemented methods for writing and reading the current budget name and creating new directories and files + changed file paths

Created and implemented getChosenBudgetCategories method + Disabled/Removed buttons depending on which budget window is shown

Savings and SavingsGoal classes with tests, implemented into GeneralBudget

See merge request !40

GeneralBudget set BudgetPeriod to be days left of month, and made FileHandling methods static.

See merge request !39