[FIX] web,base: document template widget in settings

Go to General Settings, click on 'Change Document Template', and
in the opened dialog, click on a template.

Before this rev., it automatically closed the dialog, which was
not really convenient.

This rev. introduces a new field widget (image_selection) for
this use case, and ensures that the dialog doesn't close when
a template is selected.

Test written by @mba-odoo