Fix deprecation warnings for Qt classes.

[acolwell]

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PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Fixes deprecation warnings for a few Qt classes. This simply uses default constructors for parameter defaults instead of deprecated constants.

Have you tested your changes (if applicable)? If so, how?

Yes. Code builds locally without the deprecation warnings now.

[devernay]

LGTM