[FIX] web: view manager: overlapping switches between views

The problem occured when the user quickly clicked several times on the view
switcher or the breadcrumbs. The function switch_mode() of the view manager
was called once per click. This function's role is to prepare the requested
view, and once it is ready, to remove the former one from the DOM and insert
the new one instead. It is thus asynchronous as it waits for the
fields_view_get and search_read RPCs' response.

Unfortunately, when those calls to switch_mode() overlapped, the former view
reference was lost and it was thus never removed, producing a weird situation
where two views were displayed one above the other. This may happen when a
view takes a while to load, or simply when the user double-clicks on the
breadcrumbs or view switcher.

This rev. makes sure that calls to switch_mode() can never overlap by refusing
any other call when there is still one processing.
We also added a protection against breadcrumbs and view switcher double-clicks.

opw 658924