support read-only buffers

[cmyr]

Another thing for the "eventually" pile. (A scratch buffer is not backed by a file, and does not warn (#174) the user if they close without saving. If they save it becomes a normal buffer.)

Read only buffers should be marked as such through the UI, probably the status bar?

[jansol]

TextEdit does warn if you attempt to close an window that has content but is not backed by a file yet. I think that is desirable behaviour for a scratch buffer.

[jansol]

Hmm, unsaved documents already work as expected in xi-mac. Do we want support for unsaved buffers that explicitly don't warn upon closing?

[cmyr]

Yea I don't think we need to do anything for scratch.