fixup! InterpreterPane: use MigLayout for bottom pane
InterpreterPane: narrow getComponent return type

Downstream, we will need to rely on the fact that the component is a
JSplitPane. Better to make this assumption explicit and "fail fast" if
it ever changes, rather than use casting or defensive programming
downstream which will break more subtly.
InterpreterWindow: synchronize split pane sizes

When the divider is resized for the active InterpreterPane, let's set
the dividers of all other InterpreterPanes to match, so that switching
tabs appears seamless with respect to the split panes.