Splash screen for first-time orientation

[earth2marsh]

The first time a user visits the editor, a translucent overlay should call out the main points of interest, for example (rough hack):

Before loading, the app should look for a localstorage key called something like "viewedOrientation" and not show the screen if set. Otherwise, show the screen, set the key, and dismiss on any keypress or mouse event.

Consider adding a link for "Orientation" to the help menu as well, #31 .

[mohsen1]

Done