change: handling of empty literal values
changed behaviour: setting a literal-value to empty string won't delete the according
property
affects literal and markdown widgets
add option to disable querying for labels
Fix: white screen freeze when adding a property

Bug: Create an empty graph with one class without instances. Then choose
'create instance' from the class navigation menu. In the pop over controller,
trying to add a property via the 'Add property' button creates a faded out
screen that cannot be exited by any other means than reloading the OntoWiki
main page.
prepare changelog for v0.9.7
Merge tag 'v0.9.7'

v0.9.7