Remove Qt4 stuff from 2.6 branch

[acolwell]

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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
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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)
• 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?

It removes Qt4 related code and build logic from the 2.6 branch. It also removes all conditional code inclusion based on QT_VERSION and requires that we build with Qt version 5.15.3 or greater. This greatly simplifies the code and will make it easier to do a Qt6 port. It also paves the way for simplifying our shiboken/pyside usage since we won't have to cater to the Qt4 versions of these tools.

This change does not introduce any new code. It simply removes build logic, #ifdefs, and files that are no longer needed. It essentially produces "identical" builds to what our GitHub actions for Windows and Linux were already producing. Both platforms build with a version of Qt 5.15.3 or greater.

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

Yes. I did several builds as I worked on the various commits to ensure that I did not break anything.
https://github.com/acolwell/Natron/actions/runs/8350544964
https://github.com/acolwell/Natron/actions/runs/8350727509
https://github.com/acolwell/Natron/actions/runs/8350958614
https://github.com/acolwell/Natron/actions/runs/8350963516
https://github.com/acolwell/Natron/actions/runs/8353432203
https://github.com/acolwell/Natron/actions/runs/8353465758

I also took a Windows installer from the final build and did a bunch of ad-hoc testing to ensure that everything still appeared to work. I didn't notice any issues. This testing was by no means exhaustive, but since I'm not changing any functionality I don't really expect any new bugs to be introduced by this change.

Futher details of this pull request

I split this change into commits that each have a particular focus. The intent was to make this a little easier to review. I specifically targeted Qt 5.15.x because it allowed the elimination of all the #ifdefs and it is the version that makes it easiest to port to Qt6. I also see little benefit in supporting earlier versions of Qt 5 given that we build the code ourselves and distribute all our dependencies, including Qt, with it.

[rodlie]

Great 👍

The changes looks ok to me and 5.15.3 as a minimum is also ok.

I will let Frédéric approve this.

[devernay]

Spring cleanup! LGTM!