ENH: Added hint about context menus in subject hierarchy

Context menu was said to not be straightforward, and now with the two separate context menus it is even less. Tooltips have been added that pop up when entering subject hierarchy. If there is displayable data loaded, and the user enters subject hierarchy, a tooltip appears on the displayable item about the main context menu. On a second enter into the module, a tooltip appears for the visibility menu. The state of the shown hints is stored in application settings.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25927 3bd1e089-480b-0410-8dfb-8563597acbee