Write to temporary file when saving on Android

Carsten Hartenfelsrequested to merge
hartenfels/krita:feature/bug-498257 into master

Instead of using QSaveFile, because that doesn't work on Android due to sandboxing. Nor does the usual method of QFile::copy work on Android, we have to do that manually by opening the target file and shoveling bytes.

BUG:498257 (More details about the issue can be found in that bug.)

Test Plan

  • Save a file. Observe that this works.
  • Save a file that takes very long to write. Modify it. Save it again. While it's saving, terminate the application. Observe that the file doesn't get corrupted anymore, instead it will be in its old state.

Formalities Checklist

  • I confirmed this builds.
  • I confirmed Krita ran and the relevant functions work.
  • I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
  • I made sure my commits build individually and have good descriptions as per KDE guidelines.
  • I made sure my code conforms to the standards set in the HACKING file.
  • I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.

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