Allow OpenSkyNetwork DB, OpenAIP and OurAirports DB stuctures to be
shared by different plugins, to speed up loading.
Perform map anti-aliasing on the whole map, rather than just info boxes,
to improve rendering speed when there are many items. Add map
multisampling as a preference.
Add plotting of airspaces, airports, navaids on Map feature.
Add support for polylines and polygons to be plotted on Map feature.
Add support for images to 2D Map feature.
Add distance and name filters to Map feature.
Filter map items when zoomed out or if off screen, to improve rendering
performance.
Add UK DAB, FM and AM transmitters to Map feature.
Use labelless maps for 2D transmit maps in Map feature (same as in ADS-B
demod).
