Signal actions are too hard to use, make them more user-friendly

The newly added signal event in QT3DS-4128 (onActivated, onDeactivated) for a timeline now requires creating dummy object to emit the signals. This may be confusing to the end-user, so we need to fix that by hiding the fact we are creating dummy objects.

Instead of making user to drag'n'drop Group object, for example, add an entry to context menu for adding a "Signal" that automatically has the signal action in it. This signal object can be internally just a Node with a fixed-length timeline. When entering this signal node, the signal is emitted. The signal object's timeline may only be moved, but it's duration must not be adjustable.