Change fgcommand to take an optional property tree root element.
This fixes the animation bindings to use the defined property tree root - to support multiplayer (or other) model that can bind to the correct part of the property tree.
Requires a corresponding fix in sg to allow the command methods to take an optional root parameter.
What this means is that when inside someone else's multiplayer model (e.g. backseat, or co-pilot), the multipalyer (AI) model will correctly modify properties inside the correct part of the property tree inside (/ai), rather than modifying the properties inside the same part of the tree as the non-ai model.
This means that a properly setup model will operate within it's own space in the property tree; and permit more generic multiplayer code to be written.
This is probably responsible for some of the pollution of the root property tree with MP aircraft properties.
This adds the framework for platform cursor implementations; Windows and X11 to be done.
It also extend the mouse-input code to generate hover events suitable for driving tooltips.
Note there should be no visible functionality change from this commit, since everything
is inactive until fgdata changes are made.
