[GUI] Remove Qt and make SofaImGUI as default viewer

[bakpaul]

This PR removes Qt from the sources of SOFA and makes SofaImGUI as the default GUI.

The Qt sources haven't bee pluginized yet, the repo is already here : https://github.com/sofa-framework/Sofa.Qt but I prefere to wait until this PR is merged to make the export, in case another PR modifying the sources is merged before. -> see https://github.com/sofa-framework/Sofa.Qt

[ci-depends-on https://github.com/sofa-framework/SofaGLFW/pull/141]
[ci-depends-on https://github.com/sofa-framework/Sofa.Qt/pull/1]

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By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[bakpaul]

[ci-build][with-all-tests][force-full-build]

[bakpaul]

Ci will be happy once #5234 is merged : scene tests using Qt-based code from SofaMatrix will be automatically excluded thank's to the RequiredPlugin not met.

[bakpaul]

[ci-build][with-all-tests][force-full-build]

[sofabot]

[ci-depends-on] detected during build #17.

To unlock the merge button, you must

• Merge or close Fix in tree compilation Sofa.Qt#1
  For this build, Sofa.Qt/ExternalProjectConfig.cmake.in will be edited by builders with
  GIT_REPOSITORY https://github.com/sofa-framework/Sofa.Qt
  GIT_TAG 4776bc609a05e1a09be298437cd67336598e3dfd

[bakpaul]

Last build has been performed using ci branch sofa-framework/ci#61
See: https://ci.inria.fr/sofa-ci-dev/job/sofa-framework/job/PR-5176/CI_CONFIG=ubuntu_gcc,CI_PLUGINS=options,CI_TYPE=release/15/parameters/