[GUI] Factorize camera related code to use in child class

[bakpaul]

This PR factorizes piece of code that check for the presence of a camera in the scene to be used in child class when reloading the scene.

[ci-depends-on https://github.com/sofa-framework/SofaGLFW/pull/281]

---

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

---

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).

[sofabot]

[ci-depends-on] detected during build #1.

To unlock the merge button, you must

• Merge or close Fix scale issue and gizmo not showing when reloading scene SofaGLFW#281
  For this build, the following CMake flags will be set
  -DSOFAGLFW_GIT_REPOSITORY='https://github.com/sofa-framework/SofaGLFW'
  -DSOFAGLFW_GIT_TAG='d54eb982f723000e05e6eeaafeb3d52e28ce5052'

[sofabot]

[ci-depends-on] detected during build #2.

All dependencies are merged/closed. Congrats! 👍