Issue # 893: Add Help menu

This change adds a Help menu with items to visit the OpenEmu Web site, the wiki user guide, the wiki release notes, and the GitHub issues page.

See OpenEmu/OpenEmu#893

edit:
I don't believe a long-term implementation of a help function has actually been decided, which is why i labelled this branch v1 — it's just something to start with.

Here is a screen-shot if you're lazy: http://i.imgur.com/UHdoTM4.png

(edit2: I renamed 'GitHub User Guide' to just 'User Guide', thought it would be confusing otherwise)