Hidden inspectable properties/commands

For debugging purposes, we could add a Hidden property to InspectionElement, and hidden things would only be shown when a "debug mode" is enabled (e.g. after Ctrl+Shift+D is pressed).

added model view labels

mentioned in merge request !21 (merged)

mentioned in merge request !38 (merged)

closed with merge request !38 (merged)

mentioned in commit ccbbb4fb