The docked side panel could only be closed with Escape, which was
unreliable: MainForm consumes Escape before ActionLeadIn's KeyDown
handler, so with a part selected ActionManager called the action's
empty CancelAction and nothing happened; with nothing selected the
panel closed but was stored as the previous action, so the next
Escape reopened it.
- EditNestForm side panel gets a header with the panel title and an
always-visible close button that ends the action.
- ActionLeadIn.CancelAction now steps back (unlock contour, then
deselect part), replacing the dead KeyDown handler.
- Actions can opt out of Escape-resume (ResumeOnEscape); the lead-in
action does, so a closed panel stays closed.
