Borrow new timer style from Tetravex

* Show a nice clock icon instead of the word "time"

* Show hours only if the game has lasted at least an hour. (Considering
  the game automatically pauses when the window loses focus, that means
  you must take Minesweeper far too seriously.)
Remove "use number border" preference

The option to draw borders around the numbers is excessive
configurability. It doesn't even look good. Bye.