Metatags GUI

Looks very nice, indeed.
New screen already provides added value for module users, however, to make it even more useful, I would add filter option (like in URL aliases list)

Obviously, there is demand for simplifying mass tags administration.
Please look at another suggestion on at #1241334: Auto settings for Metatags Quick

Probably, it's good to spin off nice-to-have, but optional components to the helper module, which can be switched off on production, and used only in dev/staging (similar to views/views_ui separation). What do you think?

I'll be taking a more in depth look at the GUI stuff tomorrow, but I think the path-based GUI should be part of the core module, as I think 90% of people will use it. The default Metatag settings option could stand on its own, as less people might use it.

The GUI works great for me. No issues that I can see.

Few code formatting changes, removed (deprecated now) functions that handled until now path-based tags, plus I've changed path based URL from admin/settings/search/metatags_quick/path_based_metatags to admin/settings/search/metatags_quick/path_based - it is already long, and it is clear that path belongs to metatags_quick.

Really nice work done, Sebastien!