see #6355: do not create "Imagery/More" submenu if there is enough space on screen

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6098 0c6e7542-c601-0410-84e7-c038aed88b3b
see #8921: avoid setTooltipText(null) (gives NPE when tooltip is shown on some JVMs)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6099 0c6e7542-c601-0410-84e7-c038aed88b3b