Check active theme definitions before adding regions

Problem/Motivation

Regions module allows other modules (not only the active theme) to define regions, so blocks can be added (by block or context modules). The idea is this will help with interoperable features between themes with potentially different regions. However regions.modules doesn't check whether or not the active theme has already defined a region before adding through hook_footer(). It also adds a prefix to avoid potential conflicts, which makes comparing regions difficult.

Proposed resolution

Check if the active theme defines regions before adding. Remove the custom prefix to make this easier (no risk of conflicts, since we'll be checking).

User interface changes

The region labels no longer include 'Regions:' prefix, in the core block settings and context ui.

API changes

• The project name is no longer used in hook_define_regions().
• For ui consistency, a region title is now required in the hook_define_regions().
• The full path is now required, when defining optional css and js.
• $theme_key is now an optional param in _regions_list().