#182 disallow multiple instances

[Brian Robb]

If I click a sphere file in windows explorer it opens SDE
with that file open.

If I then click another sphere file in windows explorer it
opens another SDE with that file open.

It'd be nice if the editor knew that it was already
opened, and just sent the command to the already open
editor.
So instead of having two editors editing two files, one
editor editing two files.

Project files could perhaps open a new editor though.

[Magnus Hemmer Pihl]

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Opening a files in the same editor by default is fine - but do
now disallow multiple instances of the Sphere engine
running at the same time. Doing so would limit functionality,
rather than help.
For a server application I'm writing in Sphere it's vital that I
am able to run several Sphere engines simultaneously
(interconnected through sockets). Disallowing multiple
instances would make this approach impossible.

[Brian Robb]

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I only intended this for the editor.
If you want to play two games in the engine at once, I don't
mind.

[Tim Enderling]

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Now files are opened in only one instance except for (which
was the complicating part) project files in the SDE.