[qml] PhongImageViewer: Various improvements

[gregoire-dl]

Description

Various improvements / small fixes for the PhongImageViewer.

Features list

• Remove Qt5Compat in DirectionalLightPane component.
• Use qml Loader in Viewer2D for PhongImageViewerToolbar and DirectionalLightPane components.
• Allow PhongImageViewer to read droped pictures.
• Fix loading status and display image error info pane.

Implementation remarks

Linked to QtAliceVision PR: alicevision/QtAliceVision#89

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.52%. Comparing base (8ffc283) to head (c9f0285).
⚠️ Report is 7 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2781   +/-   ##
========================================
  Coverage    79.52%   79.52%           
========================================
  Files           51       51           
  Lines         6975     6975           
========================================
  Hits          5547     5547           
  Misses        1428     1428           

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