Remove cog_platform_init_web_view() from the API

[aperezdc]

Having to call cog_platform_init_web_view() by hand is annoying. I'm sure we can change it in the future, we can add a CogView:viewport property (most of the code is already there) and then view code that needs late initialization when the view is known to have a viewport assigned can monitor the property. Currently what the x11, drm, and gtk4 plug-ins do is that (late initializations) or other configurations which could be moved to the GObject.constructed virtual function.

Anyway, this is not something we need to change now, I am writing my thoughts here so we can add an issue and make a patch later.

Originally posted by @aperezdc in #675 (comment)