Icon set management

[nohal]

• Implement management of user icons via GUI
  • Allow setting the size of vector icons (Save the user doing it in Inkscape or elsewhere, SVG optimizers also may remove the size info)
  • Allow setting hotspot
  • Allow the user to select whether the icon is scaled while zooming or keeps fixed size
• Implement management of the set of icons available for the waypoints
  • There are (too) many icons, allow the user to select a subset to be available for him in the waypoint properties combobox (and perhaps make it filterable)