Nodes inside the osg scenegraph now hold a strong reference to
the according canvas element. Canvas elements in turn now only
hold a weak reference to the according node. This simplifies
for example the canvas::Group as it does not need to keep a
list of children on its own anymore. This is now stored inside
the scenegraph (as it was already before).
The restructuring will also allow to use a canvas::Group for
the canvas gui inside FlightGear and share for example the
handling of stacking based on z-index.
Removing all event listeneres on destroying a canvas
prevents circular references due to Nasal event listeners
keeping a reference to the canvas in their closure.
Also fix event handling with direct children of the root
group and add some more helpers to the Canvas.
This allows tracking elements while at the same time changing the
rotation of both animated elements to fill the available distance
to the target element. This allows for example animating gear
scissors or other connected objects like gear doors and their
actuators/arms (possibly connected to the gear strut).
Style setters are now only setup once for each element type and
not for each element instance as before. A static map holds the
setters for each element type. Also an animation type is stored
which will later allow to animate properties of Canvas elements
without specifying and animation type.
Create a pick group for each object, instead of a single one. This
can be optimised in the future but this structure is backwards-
compatible with existing usage, which is more important.
Pick, knob and slider animations support additional object-names,
which are always invisible and untransformed (in the case of knobs/
sliders). These objects act to extend the hover/pick area, making small
switches and knobs more usable. (Especially for users with smaller
screens / windows).
