Some remarks: - `PlatformImpl.startup()` from Jav...

# tornadofx

[hastebrot]

Some remarks:
- PlatformImpl.startup() from JavaFX 8 is easily exchangeable with Platform.startup() from JavaFX 9, for future compatibility.
- It is not explicitly needed to run the toolkit launch in a new thread. And multiple calls to startup() don't throw an Exception.
- supplyStage() (for startup) and consumeStage() (for the action setup) are customizable.
- I dropped the notion of "primary Stage". It seems primary stages are only required for Java applets.
- setupStage(): Stage does not return a Future (which makes it simpler to use) but internally uses futures.