Undo button, shortcut keys, and bugfixes!

- Added an "undo" button to undo accidental presses of "DONE!" The program now keeps track of an undo stack that goes all the way to the beginning of the current session.

- Added shortcut keys. DONE: Space, SKIP: Escape, UNDO: Backspace. Sadly, I have not been able to make them triggerable even when the window is inactive :(

BUGFIXES:

- Fixed a bug in which the average, number of attempts and best time were not correctly showing.