NetworkView: add "pin" button

[bengardner]

Now that we can transfer items directly to/from the network chest, it would be nice to be able to "pin" the NetworkView GUI.

The "flib" mod has a good example of how to do that.
I rigged something up based on the flib 'test' example and found it to be handy.
I can put the Network View frame next to the Character inventory frame and move stuff back and forth.

A dialog is "pinned" by setting player.opened=nil and "unpinned" by setting player.opened back to the original value.
There is also some trickery with the callbacks, as you get an on_gui_closed event when clearing player.opened.
The 'close' buttton handler also has to be modified to be able to close the dialog when pinned.

[year6b7a]

This makes sense to me. If you have a branch that can be easily merged I'd be happy to get that in. Otherwise I can take a look at implementing this when I get a chance.