[IMP] web_editor, website: improve editor loading effects

- Share the code for tab switching loading and slow option loading.
- Now add a loading effect over the content when the content is changed
  and a loading effect over the editor panel when the editor panel is
  being changed

Part of https://github.com/odoo/odoo/pull/55614


task-2213114

closes odoo/odoo#55614

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>