Hide bottom panel when showing a SpinWidget, so we gain
on the bottom the space covered by the SpinWidget to see
more how a change is affecting the book text.
For clarity, rename delay_repaint & refresh_callback
to hide_on_apply & when_applied_callback.
Normalize SpinWidgets: have them both accept a
cancel_callback and a close_callback.
InfoMessage: call dissmiss_callback also on timeout.
b92c078e9c