0.21.0

Added

• Added all difficulty levels of 6x6 grids.
• Adds a icon on the main screen to indicate if the current game values are not calculated via the
  usual decimal numeral system. The icon will show the larger digit of that numeral system, that is
  2, 4, 8 or F (indicating the hexadecimal system). (reported by sideria)

Changed

• Re-implements the hint popup: It got more colorful and should indicate at first sight if there are
  any errors by different background color and icon. The popup is disposable via a single tap at
  any place on the screen. (idea by tuxiqae)
• A long tap on a empty cell now copies common pencil marks of the cells in the same cage.
  Works on an empty cell (no value set and no pencil marks) if all non-empty pencil marks of the
  other cells of the cage contain the same or a similar set of pencil marks.

Fixed

• A solved game could not be restarted. (reported by MageDelfador)
• Avoids statistics diagrams to show only a part of the data. (reported by mccartney)