[IMP] search views: more generic name for saved filters

Also rename variables to a more neutral term
that will hopefully remain untouched (custom filters)
regardless of the UI label.
[IMP] ir.filters: new filters are local to the menu/action by default

Allow binding an optional `action_id` to filters.
The web client will try to identify the specific
action ID when saving new filters. If no contextual
action exists, the filter is saved globally for
the model.

This will automatically keep filters within their
original menu when there are several menus/actions
leading to a given list of documents.
In some cases the action_id will not match the
filter model, which should be fine (e.g. when opening
a many2one completion popup for model `foo` within
a menu of model `bar`).

It is also still be possible to have a filter apply
to all actions/menus for a given model by manually
deleting the action_id value in the filter
(e.g. via the Manage Filters debug menu).

When updating a filter the action_id value is ignored
so that old global filters will be gradually replaced
by new "local" filters.

Also added an _order to ensure stable ordering of the
filters.