bugfix: rangechecker returns too many elements

The debug statement paid off! Found another bug in the range checker
where the number of "pre" elements was too large if there were already
pre-elements in view. This bug would have been invisible without the
debug statement.
Reverse Pointers with multiple sources

Implement context menu for back pointers with multiple sources. If there
is more than one pointer to a location, a context menu will open with
all the possible sources. Selecting a source bring you to it.

There seems to be something wrong with the context menu style - that
will be fixed later.
Goto textbox: fix formatting

Caret was getting lost whenever the text formatted itself.
Non-hex keys were accepted.