Implement wish 319522: Option to restart current game

Summary:
BUG: 319522

- Add a new action "Restart Game" between "New" and "Print" to the menu and toolbar
- "Restart Game" asks for confirmation if game is not finished yet
-  The "Restart Game" button is only active if a game is running

Related changes:
- Change the confirmation text that appears when the user clicks the "New" button to "New Game" instead of the confusing "Restart Game"
- The "New" and "Print" buttons are now only available if a game is running (on the welcome screen clicking "New" had no effect, clicking "Print" showed a message box that there is nothing to print)

Reviewers: #kde_games, aacid

Reviewed By: aacid

Subscribers: aacid, kde-games-devel

Differential Revision: https://phabricator.kde.org/D12907