Initial version of user-defined FOV export function.
Export of FOV representations is done with help of SimpleFovExport class,
which works in a following way:

1) Save current SkyMap instance state and preferences.
2) Center SkyMap instance to selected SkyPoint and set appropriate zoom (zoom
is determined in a way that ensures visibility of entire FOV and best use of
available paint device size).
3) Paint the contents of SkyMapComposite on passed QPaintDevice subclass
instance.
4) Paint overlays (except FOV symbol).
5) Disable SkyQPainter translate() transform.
6) Paint FOV symbol (if requested).
7) Revert all changes done to SkyMap.

TODO: add documentation, fix clipping and other minor TODOs.