[FIX] doc: guides section was removed, move CDN to reference
[FIX] ir_qweb: css asset: use which to find lessc and sass

Also, use lessc.cmd instead of lessc on Windows. It allows to run
lessc command with only nodejs and the less module (no cygwin).
[IMP] various improvements to the action manager (web client)

* add a replace_breadcrumb option to actionmanager
* correctly calls on_reverse_breadcrumbs when needed
* add a semi-protected way to manipulate breadcrumbs (get_widgets)
* calls do_show when changing widgets
Merge remote-tracking branch 'upstream/master'