Show Playground UI

[tischi]

@NicoKiaru
Currently the Playground Sources TreeView is already shown (in IntelliJ) when one shows the ImageJ UI, that is when executing this code and having the Playground on the class-path:

		ImageJ ij = new ImageJ();
		ij.ui().showUI();

Could we change this such that is more controllable, such that the user/developer could choose whether the Playground's Sources TreeView is shown or not? Or maybe the TreeView is only shown once the first source is added to it?

[NicoKiaru]

Done in commit 033d322

Also there's a COmmand to force show the UI

[tischi]

Cool!