D7560: Properly show/hide preview dialog

[Christian Ehrlicher]

chehrlic created this revision.
Restricted Application added a reviewer: KDE Games.
Restricted Application added a subscriber: KDE Games.

REVISION SUMMARY
  The visiblity of the preview dialog was not synced with the preview action
  
    in the main window. Also the preview image was not created when then same
    puzzle was opened after going back to the overview.
    Some c++11 changed (using nullptr instead 0)
    don't leak PuzzleComponent in GamePlay::loadPreview()

TEST PLAN
  - make sure to have a clean environment (e.g. no saved puzzles)
    - open a puzzle, activate preview
    - go back to overview
    - open another puzzle -> no preview and button state is unchecked
    - go back to overview
    - open first puzzle -> preview is open and button state is checked
    - go back to overview
    - open puzzle again -> preview does show something (instead 'Image is not available')

REPOSITORY
  R423 Palapeli

BRANCH
  preview

REVISION DETAIL
  https://phabricator.kde.org/D7560

AFFECTED FILES
  src/engine/gameplay.cpp
  src/window/mainwindow.cpp

To: chehrlic, #kde_games
Cc: #kde_games
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