[IMP] website, *: make theme custo options act as simple snippet options

* web_editor, theme_bootswatch

Instead of having an entirely dedicated system for theme options in a
third tab of the left panel, those theme options are now simple snippet
options. See the customizeWebsite generic method.

This allows to make any option available in the third tab or on any
meaningful element like the header or the footer. This also allows to
take advantage of all the features of the left panel: dependencies,
visibility update, etc.

Note: same as before, those changes do apply the color/size/layout
immediately on the website, even if not saved. Changing that behavior
is complex and might be the job of another task.

Part of https://github.com/odoo/odoo/pull/41166
task-2088298