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.
Make position finalisation happen in the same phase as scenery load, i.e as a task during the main loop, instead of during the init loop. This is compatible with the existing reset logic. Unfortunately more work is needed; the environment code doesn't update the local station quickly enough on reset. (Fixing that is next!)
Sink the 2D panel logic into FGPanelNode, simplifying
renderer.cxx and a few other places. Also allows potential
for unloading / paging of 2.5D panels in the future.
