IOService recognizes a new 3rd state of loading data

[xulman]

The IOService allows (likely among other functionalities) a client-code to be hooked into the drag&drop chain of openers. Each opener in this chain is asked if it supportsOpen() and Object open(final Location source) is called on the first opener supporting the given input.

However, the open() call expects data to be loaded that, in the drag&drop scenario, is directly submitted to uiService.show().

This PR attempts to solve the use case when the open() actually wants to use its own way of presenting the loaded data. The proposal defines a special, unique, signalling object IOService.GOVERNING_APP_STARTED that must be returned from the open() calls in such cases, that is, when 3rd party app is opened on the incoming (dropped-in) file.

This PR works in conjunction with fiji/IO#18

[xulman]

deprecated in favour of SelfShowableContent -> #473