embed-utils: Invert the logic of condition in ephy_embed_utils_address_has_web_scheme

So that we don't need to do all the string comparisons always.

https://bugzilla.gnome.org/show_bug.cgi?id=725081
embed-utils: Add ephy_embed_utils_address_is_valid

Helper method used to check if a url is valid to decide whether to load
it or perform a search. It's mostly the same expression used by
EphyWebView, but moved to a helper function. The regular expressions used
in that expression are also moved to embed-utils and they are only
compiled once instead of everytime a web view is created.

https://bugzilla.gnome.org/show_bug.cgi?id=725081
embed-utils: do not try to prepend http:// to normalize a URL if it already has a web scheme

https://bugzilla.gnome.org/show_bug.cgi?id=725081
embed-utils: Move ephy_web_view_normalize_or_autosearch_url to embed-utils

And use it only for actually typed (or pasted) text.

https://bugzilla.gnome.org/show_bug.cgi?id=725081