RE 6901 MantidEV usability

Added help menu item and linked it to initial, incomplete
help page for MantidEV.  The help page will be completed
before the release.

refs #6901
RE 6901 MantidEV usability

When the user selects an event file to load,
default names are now provided for the event, md
and peaks workspaces.

refs #6901