- 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!

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