The slider is not functional when the user click on the "Group values by
preheat temperature" checkbox, so it's better not to show it in that case.

This must have changed at some point, but could not establish when.

This makes also the point style selection appear only when points are
actually used.

Now the fastratio app can import an XSYG file, although at the moment no
curve selection available, so the first curve is hardcoded.

This expands the importTab module letting it accept a callback function,
through which additional UI elements can be added by the apps.

This concerns the doseresponsecurve, finitemixture and huntley2006 apps.