Admin UI for variable_realm module

As part of my efforts trying to integrate domain variables with multilingual variables, I started work on a generalized admin interface for variable realms. For more info see #1559970-6: Battle plan / Progress and #1307058-15: Use new Variable API for different variables per domain..

Situation

• The i18n_variable module uses hook_form_alter() to adjust system forms, annotating multilingual variables, adding a language switcher on top and a custom submit handler for saving multilingual variables to the variable_store. These type of annotations would also be useful for other modules providing realms (e.g. domain module)
• It uses hook_module_implements_alter() to ensure that the i18n_variable implementation of the hook is executed last. This makes it fairly difficult for other modules to override or adjust the forms (need to alter registry + make sure module weights are correct).
• The i18n_variable module provides an interface for choosing which variables should be treated as multilingual variables. This would also be useful to have for other realms.

Proposed solution

Generalize i18n_variable_form_alter() and the variable selection form, to work for any realm and provide a central place for configuration...

For that purpose I have created a new module "Variable Realm Admin". I am going to upload a respective patch in a minute. For some screenshots (domain+variable usecase) see #1559970-6: Battle plan / Progress.

Concepts/ Implementation details:

• Extend variable_realm_info with any information required to build the admin interface.
• A variable realm "has settings", if variables need to first be selected as being "available for the realm". Realms which do not have settings are not handled/affected by the module (e.g. the global/default realm).
• A variable realm can have a "settings ui", which means that there will be page for selecting variables. TRUE by default, but can be disabled on a per-realm basis if needed. The list of variables available in the selection form is limited by a 'variable key', e.g. "localize" for multilingual variables, which needs to set to TRUE on respective variables.
• There are two system variables for each realm, which store the list of available variables:
  • *_conf keeps the list of high-level variables; can be modified by the end-user via the settings ui; only exists if there actually is a settings ui (see above).
  • *_list keeps the list of low-level, actual variables names; this one is used as source of information about which form elements to annotate etc. It is automatically constructed from the *_conf variable, whenever that one is saved. There is a hook for other modules to modify it.
• hook_form_alter() adds an infoblock at the top of every systems form which contains a realm-variable (as specified in respective *_list variables), and adds a set of links for switching the current realm for editing variable values.
• The realm switcher is built automatically from the "keys" parameter in the realm info array; modules can create custom implementations by declaring a "realm switcher callback" in the array.