Add checkbox to main menu to toggle hatched background rendering of areas outside of the downloaded areas

This is useful, e.g, for mapping public transport relations when one
downloads a relation modify by id and missing stops by bbox.

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6499 0c6e7542-c601-0410-84e7-c038aed88b3b
Add Javadoc for `LatLon`, add unit tests for `LatLonDialog` (parsing)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6500 0c6e7542-c601-0410-84e7-c038aed88b3b