- 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.

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

No longer load context_ui_admin.inc on every page load. Standardized format for space, key, value strings (numbers, lowercase letters and underscore only). More concise test.