Ie8 fixes 8713

This fixes a couple of issues in IE8:
See: https://trac.openmicroscopy.org.uk/ome/ticket/9771

To test (on IE8):
Right-click menu on the P/D/I tree - check that disabled items appear disabled and that sub-menus of disabled items  look OK too.
Check that P/D creation works OK, new P/D inserted into the tree etc.