#142 NavigatorView: add related entities lazily

[lifesaver2000]

In general, the idea is to be able to add at least a
spouse, parent or child by double clicking the
appropriate icon in the navigator view. Since the
current navigator works by single click, this added
capabilty wouldn't have to change existing behavior for
the user, but would allow a more graphical interface
for adding new relatives to a currently displayed INDI.
Of course, some filtering of input might be needed, for
example not allowing the addition of extra parents.
This would be an especially useful shortcut for those
who prefer the mouse over the keyboad. Since this is
true of many of today's typical computer users, I could
see this becoming the most commonly used method for
creating new relationships.

[Nils Meier]

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makes sense - if the user clicks on say 'father' of a person
and that person doesn't exist then a prompt could ask
whether to create one.

double click won't work though because we're using standard
buttons - could be an extra item in the popup though.