Deprecate usage of qSort in ToolButton

[YakoYakoYokuYoku]

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• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

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)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Beginning with Qt5 most of the offered methods in QAlgorithm that have a corresponding STL counterpart in algorithm are deprecated. This PR substitutes the qSort usage in Gui/ToolButton with std::sort.

Show a few screenshots (if this is a visual change)

N/A.

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

By building and running the Natron GUI.

Futher details of this pull request

N/A.