Allow second engine to analyze too

Allow second engine to analyze too

Clicking the Analysis menu when analysis is already in progress will
now toggle the secon engine to analyze the same position. For now,
this lasts only until the position changes (or you leave analyze mode),
after which the second engine is always switched off, and the Engine
Output window returns to single pane. Each pane now has its own header
line; the exclude header is suppressed in the second pane.