[IMP] change webclient structure: remove table

This is a small change if measured in lines of code, but is a large
change: it removes the table structure from the web client and use divs
instead.  It will likely break a lot of things (some css selectors
will not work), but it should simplify our lives in the future:
closer to being responsive, easier layout computations for the browsers
(and hopefully less discrepencies between the results), simpler html, ...
[FIX] fix display issue in IE in kanbans

vertical text in folded kanban groups did not have a correct
orientation in IE.

Also, stupid sass compiler insists on normalizing color codes. No idea
why
[FIX] properly destroy client widgets (web client)

The new action manager only destroyed viewmanager widgets instead of
any old widget.  It now checks for the widget class.

Also, minor improvement to clear_widgets: vms used to stand for
view_managers, but it can destroy any widget.
[FIX] hide 'add to dashboard' in mail menu

add to dashboard does not work with messaging client actions. So,
better to hide it than to have crashes.