Help menu and About dialog

[toloudis]

Initial add of a help menu and about dialog, to get us going. This addresses most if not all of #142

Help menu consists of

• link to image.sc forum's agave tag
• link to agave issues board to report bugs
• link to agave documentation
• link to main agave github page

The about box also contains some information about how to cite AGAVE in publications.

There are still some possible to-do's here such as license information (or links to it).

Screenshot from macOS:

[toloudis]

After some tweaks, here are the new dialogs on macos: