CellImpl make use of Persistence

[Namdrib]

0ef9a22: "need to have a way for cellimpl to refer to persistence.getMarks()"

Because right now it hardcodes all Cells to have marking as true, regardless of what has been loaded from the database (from #19). This causes the marking status to be out of sync with the menu options.

GamePanel has direct access to a Minesweeper object, which can in turn access the Persistence object. This may be bad programming, but may be how CellImpl can do this.

Need to think of a better way to have Persistence access.