@URL nodes to be "commands" when executed with control+b (Specially during Screencasts)

[Fidel]

One step in order to make Screencasts both interactive and able to teach outside Leo, would be for leo to open @URL nodes while in a screencast.

For this, when a @URL node is executed as a command, (control-b), Leo will use the web or file link as a launch-command.

Then there is the question on wether or not the @URL nodes should also be able to include commands.

Anyway, for starters, this will allow us to use screencasts not only for teaching but for automating
-we can put several @url which will be oppened when we do the screencasts

[Edward K. Ream (edreamleo)]

Good idea.