Multiple selection disables "visibility" flag

Git commit: 17a2ec0 on Windows 7/64

- Make sure inspector is open
- Select a visible and an invisible notehead (or other elements of the same kind) at the same time
Actual result: The "Visible" flag gets disabled
Expected result: the "visible" flag is set to the "neutral" state, but not disabled - clicking on it should either set all selected elements to invisible or visible.