Platform detection is inconsistent

[hinerm]

Platform detection appears to be undocumented, but influenced by launchers present.

This means I can, for example, activat the linux platform on a Windows host by the presence of a ImageJ-linux## launcher in my Fiji.app basedir.

However, Contents/MacOS/ImageJ-macosx does not appear to activate the macosx platform in the updater, meaning by default I cannot upload a new mac launcher from windows/linux.

I discovered a workaround to this where by placing an ImageJ-macosx file in the Fiji.app basedir (instead of Contents/MacOS) the macosx platform is activated... although it then (appropriately) tracks the launcher file in Contents/MacOS/ImageJ-macosx.

So clearly there is a disconnect somewhere in the launcher, and I'm not sure why this bit of code is failing to catch the Contents/MacOS launcher..