save last target, use it for livecoding UChainGUI feature

How about this for saving the last used target.

test code:
```
(
Udef(\test, {
	UOut.ar(0, WhiteNoise.ar() )
})
)

(
Udef(\test, {
	UOut.ar(0, SinOsc.ar(1000) )
})
)

(
t = Group();
"using group % for target".format(t).postln;
x = UChain(\test, \stereoOutput).ugroup_(\test).prepareAndStart(t);
UChain(\sine, \stereoOutput).ugroup_(\test).prepareAndStart(t);
y = x.gui;
y.autoRestart_(true);
)
```