[Viewer] Viewer: Fix various issues with the 2D Viewer

[cbentejac]

Description

This PR fixes the following issues:

• An issue introduced with [Viewer] Display error labels when an image cannot be loaded #2250 that disabled the 2D viewer's color picker.
• An issue introduced with [Viewer] Display error labels when an image cannot be loaded #2250 that never updated the loading status of the Panorama Viewer.
• A MaterialIcon that was not displayed anymore for the Panorama Viewer as its name had been updated by [MaterialIcons] Add script to generate the list of available MaterialIcons and update it #2247.
• Several QML warnings that appeared when trying to access the Float Image Viewer's item status without checking if it existed beforehand (introduced with [Viewer] Display error labels when an image cannot be loaded #2250).
• An issue introduced with [Viewer] Display error labels when an image cannot be loaded #2250 that prevented the Sequence Player from updating the image currently displayed in the viewer.
• An issue introduced with [Viewer] Display error labels when an image cannot be loaded #2250 that blocked any update of the resolution label.
• An issue introduced with [Viewer] Display error labels when an image cannot be loaded #2250 that caused the "loading" icon to never be displayed when updating the image in the 2D viewer. This specific problem also requires [qtAliceVision] FloatImageViewer: Correctly set the LOADING status QtAliceVision#64 to be fixed.