Fix bug #420 - The Add keyframe button shouldn't be active if there is not timeline
button status updated on signal canvas_interface->signal_rend_desc_changed
Doc Comment Generator - Class : Dock_Keyframes
KeyframeActionManager : little bug hunt (resolution genete)
- KeyframeActionManager::time_changed_connection assignement was wissing
- test before KeyframeActionManager::selection_changed_connection & KeyframeActionManager::time_changed_connection disconnect