Load the fields_view of all views of the action together. If a searchview is
required, also load its fields_view and its filters.
Use lazy-loading for fields as they are required for the Pivot and Graph
views only, and may be used by Filters and GroupBy menus of the Search view
if the users clicks on 'Add a custom Filter/Group'.
The fields are thus also loaded directly if the first view of the action is
Pivot or Graph, and otherwise, they are lazy-loaded when/if the user opens
a Pivot or Graph view, or if he wants to add a custom Filter/Group.
Some refactoring of views API as well:
- Views must now be instantiated with their fields_view
- Move code requiring fields_view from start() to init(), or willStart() for
heavy processing.
- Remove view_type attribute from views as it was used to call
fields_view_get.
- Remove view_id parameter to views' init as it is no longer used.
- Move duplicate code from various views' init to the View widget.
- Remove view_loading() function from views as they now follow the classical
lifecycle of widgets (init -> willStart -> start -> destroy).
- Searchview now extends View like all other views.
- Remove useless function guard_active() as it was used for the form_view's
action buttons to wrap handlers to ensure that they are executed only if
the form_view is the current active view. This is not useful anymore as
from v9, views and their control panel are appended in the DOM
simultaneously, so one can't click on a button of a view which is not the
current active view.