Namespace globals canvas map

map.nas - 	provide a high level method to create typical maps in FlightGear (airports, navaids, fixes and waypoints) for both, the GUI and instruments
implements the notion of a "layer" by using canvas groups and adding geo-referenced elements to a layer
layered maps are linked to boolean properties so that visibility can be easily toggled (via GUI checkboxes or cockpit hotspots)
without having to redraw other layers

GOALS:	have a single Nasal/Canvas wrapper for all sort of maps in FlightGear, that can be easily shared and reused for different purposes

DESIGN:	... is slowly evolving, but still very much beta for the time being

API:		not yet documented, but see eventually design.txt (will need to add doxygen-strings then)

PERFORMANCE:	will be improved, probabaly by moving some features to C++ space and optimizing things there


ISSUES:	just look for the FIXME and TODO strings - currently, the priority is to create an OOP/MVC design with less specialized code in XML files


REGRESSIONS:  744 ND: toggle layer on/off, support different dialogs

ROADMAP:	Generalize this further, so that:

- it can be easily reused
- it uses a MVC approach, where layer-specific data is provided by a Model object
- other dialogs can use this without tons of custom code (airports.xml, route-manager.xml, map-canvas.xml)
- generalize this further so that it can be used by MFDs/instruments
- implement additional layers (tcas, wxradar, agradar) - especially expose the required data to Nasal
- implement better GUI support (events) so that zooming/panning via mouse can be supported
- make the whole thing styleable

- keep track of things getting added here and decide if they should better move to the core canvas module or the C++ code


C++ RFEs:
- overload findNavaidsWithinRange() to support an optional position argument, so that arbitrary navaids can be looked up
- add Nasal extension function to get scenery vector data (landclass)
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