Show data set selections in 3D window

Selections in the data set overview UI now show up in the 3D window,
which should make it more convenient to switch between one of several
loaded files.

* DataSetUI class, to wrap up the user interface a bit

* Various changes to 3D view to display only the current selection.

* Remove $FILE_LIST usage from shaders, since it's more or less subsumed
  by the new interface.