[IMP] kanban, web: kanban and form view kanban selection widgets can

now be dependent on the current stage of the record. Also tweaked
the tooltip of kanban group header.

his is done by adding some options and improving the first implementation
(never used) of this feature :
- added group_by_tooltip option on field used to group on kanban view. Its
values is a collection of fields on the record used to group. The tooltip of the
group header will be the text formed from those fields values, allowing custom
tooltips for different kanban columns.
- added states_legend option on kanban selection widget for kanban and form
view: used to change the selection value by a value coming from the record
used to group_by (kanban) / states_legend_field (form view). This allows
customizing the kanban selection widget depending on stages.

Future commits will aim at using those features in the project,  hr_recruitment
and crm addons.