[sugar] fix for 1846, sugar crashes when nick is whitespace

[Owen Williams]

Here is a patch for http://dev.laptop.org/ticket/1846.  I am just
patching intro.py to reject usernames that are all whitespace and to
trim excess whitespace on either end.

Also, when a user supplies a blank nickname and presses the forward
button, I briefly flash the "My Name:" label.  This can be changed if
there's a better way to indicate why the interface is refusing to
advance.

owen
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