[IMP] base/web: XML templates are added into the asset bundles.

XML files are now declared in python module manifests. During the qweb
't-call-asset' directive, assetbundle will fetch the declared xml files,
apply the inheritance (t-inherit) and create a javascript service (for
eg: 'web.assets_backend.bundle.xml') which is added at the end of the
*.js mimifier file.

When the debug mode is activated, comments are added in the template
indicating which file the template comes from as well as the
inheritances applied to it.

****

JavaScript:

assets.js (module @web/core/assets) takes care of loading libraries,
javascripts and styles.
`loadJS(url)` (loads the javascript and returns a resolved promise when
the templates are also loaded via the '*.bundle.xml' service)
`loadCSS(url)` (loads the style a resolved promise when the file is
loaded)
`loadXML(xml, app=assets.defaultApp)` (load template into
application/owl, used by the `*.bundle.xml` services)
`getBundle(bundleName)` (get the bundle descriptor)
`loadBundle(desc)` (load the files and bundle from a descriptor)

templates (XML element content all owl templates)

A new `ready(serviceName)` method on boot.js lets you know when a
service is loaded are the require.

The xmlDependencies attribute no longer exists.

Python:

The xmls taken into account by assetbundle.py, applying `t-inherit`
inheritances and adding an `name_of_the_bundle.bundle.xml` service in
the generated JavaScript file.

****

Every manifest changes is into the next commit, except 'web_tour' in
this current commit as example.

Part-of: odoo/odoo#95500