Fix the bug in which right click and then delete key would no longer work to delete objects

Navigation handling without a currently selected menu entry now only happens if key is up or down arrow
(mind the space changes)
Remove attachments from the avatar's attached list before destroying them.
This should prevent the idle callbacks from accessing deleted attachments when looking for the CCS signature.
Merge branch 'master' of https://github.com/MelanieT/SingularityViewer
Merge branch 'master' of git://github.com/Lirusaito/SingularityViewer