Make FeaturesViewer available in Viewer2D to display the feature points of the current view, based on the current FeatureExtraction node.
* FeaturesInfoOverlay: overlay that displays info and provides controls over a FeaturesViewer component.
* ColorChart: color picker based on a set of predefined colors.
* display selectable-by-mouse source filepath
* add radius property to FloatingPane to control border radius from outside
* use FloatingPane in ImageMetadataView
* cache last loaded image and display it while the main Image is loading to avoid empty screen
* fix the BusyIndicator by dynamically creating a binding on the running property (Qt bug ?)
* new footer that displays the number of images / allow to change thumbnail size
* smaller sourceSize for thumbnails (faster to load)
* avoid Nan scales in Viewer2D when no image is loaded
* ImageGallery displays images from CameraInit.viewpoints
* support drag&drop of images
* Viewer2D displays and allows basic manipulation of currently selected image
