Freetype and libPNG detection in QT5

Summary

Bring back a patch that has been removed in 3597436e. The patch has been adapted to be compatible with QT 5.14.1

pkg-config is used in priority when trying to find the system QT5, which fails to find the superbuild version and use the system one instead.

Don't merge this right now. There might be other things to fix for the QT5 superbuild.

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
• Optionally, run git diff develop... -U0 --no-color | clang-format-diff.py -p1 -i on latest changes and commit