MT 5.2.6 fixes the problem where an `id` column must exist, therefore negating the need to create the `id` column.

It's easier to mandate MT 5.2.6 as the minimum required version than to fix the various upgrade problems surrounding adding an `id` column to such an active table. (Notably: Oracle doesn't seem to like having an `id` column where the contents aren't auto incrementing, and upgrading an active PQ job causes it to botch the upgrade.)
Typos.
The PQ Manager is only visible at the system level in MT5.
Update the messaging about changing priorities to better reflect truth when using the Publishing Priorities plugin, too.
Support mass delete/change priority by recognizing the "select all xxx items" link in the Listing Framework.
Bump to version 1.2.1.