IntervalTimerApp.java - change fontsize display & border

change fontsize for displayed countdown to 72
set border vor visualTimerPanel to EmptyBorder(5,5,5,5)
IntervalTimerApp.java - change buttons

adjust width & height of buttons and their font
changed frame to not resizable, deleted copyright comments
IntervalTimerApp.java - change to BorderLayout

changed content pane to BorderLayout
IntervalTimerApp.java - implemented countdown-list

+ changed layout a bit