WIP: Add OpenMW-CS help menu

This MR will (possibly) close issue #3430 (closed). I think more links to useful resources could be added. Any suggestions would be helpful.

My end goals is to take the online documentation that we have and make it available offline, but I'm still trying to figure out how to make use of Qt's QHelpEngine class. Consider this MR a first step on the path to that end. Using QHelpEngine will also affect more of the codebase, possibly requiring new classes and editing of the build files. I'm willing to take on this larger project, but I will need some help.