Added a check which allows prefix providers to limit the prefixing methods that can be used with their prefixes

- Renaming context_exists() to context_isset(). Context_exists() retained for backwards compatibility.

- Adjusting array structure of hook_context_items() to directly map to
FormAPI.
- Moving all admin-related functions, including page callbacks, into
context_ui_admin.inc.
- Cleaning up comment style.

select and specification of setter/getter type.
- Removed context_ui_block_hack() -- we are now directly overriding
theme_blocks by unabashedly taking the phptemplate_blocks function override.
Themer beware!

readability.
- Added context_ui_context API functions from _admin.inc
- Added theme_context_ui_node_links() which provides contextual node add links
based on the active context and user permissions.
- Fixed a bug where local tasks were rendered improperly / caused strange
behavior because of improper grafting of items into the menu tree.

Because of issues when using multiple themes we're going to assume that phptemplate_blocks is *the* function name we want to use.

database
- Improved admin UI layout for clearer distinction between context
setters and context getters

Fixed a bug where local tasks weren't rendered if a page call back that wasn't the default local task set a menu context