End game if too much clicks.

Game ends if clicks-progress-bar is empty.
Tile.display auto-disables if game is over.
Moved end-game related functionality into 'State.End' object.
Moved View.repaint into State object.

View.repaint was only used by State object.
  Made it a private method of State.
Refactor tile-click behavior. Divide and conquer.

Separate functions to handle
  click-to-start-game and
  click-to-continue-playing.
Delete unused method: Timer.time().
Added Level.difficulty() method.

Extracted a method from max_clicks calculation logic.
  Named this method Level.difficulty.
  Reused this method to calculate max_time.
Timer uses milliseconds and floats.

Instead of discrete seconds, Timer now uses milliseconds in floats.
This will be used to achieve better accuracy in score calculation.