Add hook to allow altering $links in DropdownLanguage block class

Patch to:
- Add call to moduleHandler->invokeAll for new hook_dropdown_language_alter_links hook.
- Add dropdown_language.api.php to document new hook.

would you be able to outline the advantage of this hook? simply, what is your use-case that you need to alter the like that you can not 'use custom title' not be useful??

also: needs tests.

@skaught certainly.

In my case all child pages belong to properties.
Properties can enable and disable languages manually via a field.
Translations may get added for child pages at any point, however until we want to enable a language for a property none of the translations for child pages should show.

This means I need to apply custom logic specific to my application to alter the available links.

Also there are certain languages that we always want to show even if translations don't exist.

This patch alone will not achieve that as link will have already been unset by the time the alter hooks runs - so I think I'll create another issue for this too.