small cleanup
GraticulePlugin: Use GeoGraphicsItems for rendering the grid
Allow for icons in icon style

* If an icon is set in icon style for a GeoPointGraphicsItem
  draw it at the point's position
Allow for settings a font in MarbleGraphicsItems
MeasureToolPlugin: Port to GeoGraphicsItems
GraticulePlugin: Set default grid alignment to Corner

* This setting lets the plugin behave more like it did before
  the port to GeoGraphicsItems
Add GeoTextGraphicsItem and GeoRectGraphicsItem
Add GeoPixmapGraphicsItem
WIP: Port APRS plugin to GeoGraphicsItems