Added login for admin and logout functionality, missing add- and delete user

src/main/java/com/application/DB/Account_handler.java

src/main/java/com/application/DB/Constants.java

...ain/java/com/application/GUI/PopUpWindows/LoginPopup.java

target/classes/com/application/DB/Account_handler.class

target/classes/com/application/DB/Constants.class

...classes/com/application/GUI/PopUpWindows/LoginPopup.class