[IMP] kanban, web: kanban tooltips and kanban states tooltips improvements.

Kanban and form view kanban selection widgets can now be dependent of the
current stage of the record. The displayed legend can be taken directly
from the stage instead of the field selection values.
- added states_legend option on kanban selection widget for kanban and form
view: used to change the selection displayed value by a value coming from the
record given by group_by (kanban) / states_legend_field (form view). This allows
customizing the kanban selection widget depending on stages.

Improved the tooltip of kanban groups customization by improving the first
implementation of group_by_tooltip parameter in kanban view declaration.
This is done by adding some options and improving the first (still not
used) implementation:
- 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.
- now all data of the kanban columns are fetched in one call instead of doing
one call for each column. The various fields to read are collected by
introspecting the view definition.

Future commits will aim at using those features various addons, notably
in recruitment, project and crm.
[FIX] kanban: fixed condition evaluation when reading stage data: ensure you are grouping on a m2o, fixed condition
[FIX] view_form: kanban selection widget: fixed issue of binding on this instead of self
[ADD] ir.qweb FileSystemLoader
[REM] Removed QWebContext.templates cache

Now rely solely on ir.ui.view#read_template()'s orm cache
[IMP] Replace gantt chart library with dhtmlxgantt 3.1