Containers which create internal panels are always stretchable

Hello! When I say something like this:

```
(cond-view
[@foo? (text "foo is set!")]
[else (text "no foo here!")])
```

then internally it will instantiate an `if-view%`, which in turn calls:

```
(new gui:panel%
[parent parent]
[min-width #f]
[min-height #f]
[stretchable-width #t]
[stretchable-height #t]))
```

There's no way to inject your own arguments for this panel. That is, if you use a `cond-view` (or any of the similar containers), you can't have a non-stretchable containee. Even if you have a simple 1-line label (which isn't vertically stretchable), putting it in a `cond-view` causes it to become vertically stretchable, which is not what I want.

Whenever the library creates secret automatic panels, they should either mimic the layout arguments of their containees, or allow me to explicitly pass arguments for them.

Thanks!