This commit introduces a new semantics to define action and method buttons
in the list view.
The aim of this feature is to be able to do some actions on selected records, or on the entire
records' domain with the help of buttons located in the controlPanel, much like the form view header buttons.
To define such buttons, one must add a `header` node in the list view's xml definition
```
<tree>
<header>
<button type="action|object" name="actionId|methodName" string="button's text" context="{}"/>
</header>
<field name="fieldName" />
</tree>
```
task-2246383
Lucas Perais (lpe)
49d9771c56