Debug plugin can display logs.
Made the mode option hidden. Can be changed to 'list' once the platform is updated.
Set the default category to 'deprecated' because that seems sensible.
alphabetized language file
Improved description for 'categories' field.
Merge branch 'plg_system_debug-logs' of github.com:okonomiyaki3000/joomla-cms into plg_system_debug-logs

Conflicts:
	administrator/language/en-GB/en-GB.plg_system_debug.ini
	plugins/system/debug/debug.php
	plugins/system/debug/debug.xml
Merge branch 'plg_system_debug-logs' of github.com:okonomiyaki3000/joomla-cms into plg_system_debug-logs

Conflicts:
	plugins/system/debug/debug.php