Port usage of KUrl and friends to equivalent constructs with QUrl.

Summary:
KUrl is part of the deprecated kdelibs4support API.
Porting away from it removes one obstacle towards legacy free sudoku for KDE.

Test Plan:
Using XDG_DATA_DIRS to point to locally built copy of ksudoku game assets, I tested the following:

  # Generate a Killer type (i.e. a custom Sudoku variant) puzzle using defaults for difficulty/symmetry
  # Reconfigured toolbars to add the Home Page action & invoke the Home Page action
  # Save the puzzle/game file & Quit the application
  # Reload the application and load the previously saved puzzle.

Reviewers: #kde_games, ltoscano, stikonas

Reviewed By: ltoscano, stikonas

Subscribers: stikonas, ltoscano, #kde_games

Differential Revision: https://phabricator.kde.org/D5544