[js/widgets] fix the InOut widget with modern jQuery versions
Several things are done there:
* reduction in size and complexity of the code
* the unused defaultsettings are removed
* the initial `unlinked` list is now correctly populated from
python-side
* the unit test is adjusted because it tested an irrelevant
implementation detail which is no longer true (but the widget of
course still handles correctly the linkto information)
Tested with ie7, ie9, chromium, firefox.
Tested with jQuery 1.6 (cw 3.17.x) and 1.10.
Closes #3154531.
