GUI-524: Hide drop-down menus when modal dialogs display

Fixes https://eucalyptus.atlassian.net/browse/GUI-524

Also fixes:
https://eucalyptus.atlassian.net/browse/GUI-528
https://eucalyptus.atlassian.net/browse/GUI-531
https://eucalyptus.atlassian.net/browse/GUI-534
https://eucalyptus.atlassian.net/browse/GUI-538
https://eucalyptus.atlassian.net/browse/GUI-539
https://eucalyptus.atlassian.net/browse/GUI-543
https://eucalyptus.atlassian.net/browse/GUI-546