Re-organize the sub-modules

Once that all the version 6.x-1 code will be ported to version 6.x-3 (which should happen on tomorrow morning, or 24 hours later), I will start to implement this.

The code has been ported from branch 6.x-1 to 6.x-3. The attached patch doesn't include the files that have been added.

I am changing the title to better reflect the task that needs to be done.

Rather than splitting the modules between basic and extra, it would be better to split the modules basing on the type of meta tags handled. nodewords.module should handle the meta tags that most probably are required from web site, nodewords_dublin_core.module should handle the Dublin Core meta tags, and nodewords_site_verification.module should handle the site verification meta tags.
nodewords_custom.module will be the module that allows the admin user to create custom meta tags without to write PHP code.

Considering the last development of the code, the modules will be re-organized as follow:

1. nodewords.module, nodewords_basic.module, and nodewords_extra.module will be kept. The difference is that nodewords_extra.module will handle some meta tags that are equivalent to the Dublin Core meta tags (in example the meta tags DATE, CONTRIBUTOR, etc); the Dublin Core meta tags will be handled by nodewords_dublin_core.module.
2. nodewords_dublin_core.module, which will handle all the Dublin Core meta tags.
3. nodewords_verification_tags.module, which will handle the verification meta tags as it is doing now. It will also allow to define more verification meta tags of the same type, in the case a user has more than verification meta tags for Google Webmaster Tools, in example.
4. nodewords_path.module, which will add the settings page for custom pages known by their relative path. This would allow to add an import script to import the settings from nodewords_bypath.module (see #657388: Migrate settings from nodewords_bypath, where DamienMcKenna wrote an update function that would allow that).
5. nodewords_custom.module, which will implement the features as I reported in my previous comment.

It is necessary to keep separated nodewords_basic.module, and nodewords_extra.module from nodewords.module because users with permission to administer meta tags can see the edit form fields of all the meta tags implemented.