This class contains validation code shared by ConfirmQuit and
URLSegmentList, that is it checks for duplicate values and compares
each value to valid_values.
Also known as Ctrl-A/Ctrl-X. You can now specify which parts of the URL
should be searched for numbers.
The setting is general->url-incdec-segments and it's a set with valid
values of 'host', 'path', 'query' and 'anchor'.
This is needed for interpolation since this change in Python 3.4:
https://hg.python.org/cpython/rev/267422f7c927
This broke qutebrowser in Debian experimental when updating python from
3.4.3-8 to 3.4.3-9 as they pulled from hg.
Fixes#968.
