Add a context menu to the GPS position in the image info window

[RKrahl]

The Image Info window in the imageview GUI contains a GPS position entry. Its value is linked to the corresponding position in OpenStreetMap. Should add a context menu that opens on right click on the link. A preliminary list of items in this menu includes:

• Open position on OpenStreetMap,
• Copy OpenStreetMap URL,
• Copy position.

The last entry would be particularly useful. It should copy the position to the clipboard in a format that is suitable to be used in the --gpspos argument to the photo-idx and imageview command line scripts and in the Filter options dialog in the GUI.