Split weston debug from weston compositor

This is a pre-cursor to !144 (merged), with more reasons for it here: !144 (comment 145234).

This series:

• decouples weston_debug_compositor from weston_compositor
• removes from weston_compositor, weston_debug_compositor reference
• install weston-debug header into include/libweston

No functional changes but:

• renames weston_debug_compositor to weston_log_context
• renames weston_debug_scope to weston_log_scope
• renames all functions that worked previously on weston_debug_scope to weston_log_scope as well