FilterWizard: don't concatenate 'http://' to the helpurl passed in

A full, working URL should be passed in otherwise this assumes a
protocol-prefixed URL is never passed since it would prepend an
additional 'http://' string to the URL which breaks it.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
FilterWizard: fix various bugs and UI tweaks

The filter design and apply buttons should now be grayed out when the
user shouldn't be able to use them.

Signed-off-by: Tim Harder <timothy.harder@analog.com>