DEV-41253: Updated the help context calculation code so that it will

now search through a panel's children to see if there are any visible
tabs that have a help topic string defined. If so, we use this string.

Updated all of the XUI files that include a tab_container to define
help topic strings for their child panels. I named all of these strings
with the floater name as the prefix and "tab" at the end. For example,
"preferences_display_tab" or "people_nearby_tab".