Add option to launch a stated machine without providing a piece of media

[TomHarte]

As presently formulated the emulator is launched by providing a piece of media. It runs an analyser on the media to determine target machine and machine configuration, then launches that machine with the media inserted.

It is a user request that the emulator also be launchable by specifying a machine (and configuration), more like a classic emulator.

[TomHarte]

This likely pairs with #311 as a unit of work, since it implies some sort of introspection into permissible machine configurations if that's going to be offered on the command line.

[TomHarte]

Scratch the above comment; #311 did not imply introspection. So does not prima facie contribute here.

[TomHarte]

#400 added this option for the Mac; I won't consider it done until it's cross-platform.

[nekoniaow]

Tested the dialog today on macOS Sierra.
It works nicely.

I would just add that it may be unintuitive for casual users to have to reach for the "New..." menu entry to get presented with a choice. So maybe presenting that dialog automatically could be a viable choice.

[TomHarte]

Agreed; applications that open only to the menubar are very OS X v10.4. I think that the Pages model is probably smartest — at first launch if there's nothing to restore then show an open file dialogue with a 'new machine' button in the lower left. Time to remind myself about NSOpenPanel, I guess.