Use the downloaded documentation from within the app #30

It would be very handy if the Plotdevice app would allow the documentation to be installed on your machine, i.e. the app would keep track of the download (and possibly latest version), and the Plotdevice Manual function under the Help menu opens that locally stored documentation in your browser.

(And if not the automatically updated download of the documentation, than at least make the menu item configurable to point to a file:/// location which the user can set).