[IMP] web,base: put buttons in list group headers

It is now possible to put buttons in the list view group headers.
When the view is grouped by a many2one field, those buttons
appear next to the header title when the group is opened.

The buttons are specified in the views in a <groupby> tag in the
list arch, with the following structure:
    <groupby name="groupedField"> <!-- must be a many2one -->
        <button type="object" name="my_method" string="Button1"/>
    </groupby>

It is also possible to add `field`, inside the `groupby` which can
be used for modifiers. These fields thus belong on the many2one
comodel, like:
    <groupby name="partner_id">
        <field name="name"/> <!-- name of partner_id -->
        <button type="object" name="my_method" string="Button1"
		attrs="{'invisible': [('name', '=', 'Georges')]}"/>
    </groupby>

These extra fields are fetched in batch when grouping on the field.

Part of task 1915702