Show the option of starting a new game on game over

When the 'Allow KMines reset' option is enabled, the game now offers to start a new game (new mine layout), in addition to restarting the same game. This is inspired by the Win7 version, and I find it very useful when trying to find a 'good' game to start playing.

Without this feature, the player has to first dismiss the 'Reset?' dialog, then choose the 'New Game' action. With this feature, the player can simply press 'S' (or other mnemonics resulted in other languages; the same strings as libkdegames are used on the buttons to avoid introducing too many i18n strings) to start a new game.

The commit fff3075c is implicitly reverted in this commit, because KMessageBox does not have a three-button option; but tried to preserve the KGuiItem semantics.