fixed scrolling the view to accomodate the selected landmark when it is grabbed, which resulted in unwanted moving of the landmark

fixed incorrect correction of new landmark position which allowed for the landmark to be place 1px beyond the bottom and/or right image boundary

added a center pixel for the landmark vis + hide the cursor when moving a landmark so that it does not obstruct the view

bidirectional reaction of selecting landmark either directly in image or through the landmark list + panning the view to ensure the selected landmark is visible

added tool widget for the `Landmarks` tool to choose which landmark annotation to add in the current photo

added class AnnotationDelegate (and subclass LandmarkDelegate) that provides common editing and viewing behavior for a class of annotations (e.g. landmarks/keypoints, bounding boxes, polygons)

replaced the implementation of `LabelTreeModel.find_index` with Qt's `QAbstractItemModel.match` function