For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
Until now mouse right-click could only select and deselect the highlighted
feature to "lock" vertex tool (and numerical editor) to it, so that it is
easier to focus only on editing of the particular feature. It was however
still difficult to pick the right feature in case there were multiple
features in one location or very close to each other. This is now solved
by the fact that repeated right button clicks will loop through the editable
features. So if there are two features in one location (A, B) then repeated
right-clicks will select: A - B - nothing - A - B - nothing ...
