buildSessionTree() and updateSessionTree() now save/restore
Window.activeFocusItem instead of only promptInput.activeFocus.
This prevents sessionTreeModel.clear() from pulling focus out of
open modal dialogs (NewSessionDialog, NewAgentDialog).
Also add forceActiveFocus() on dialog open so the first field is
immediately focused.
createAgent now returns the new agent ID so the dialog can immediately
switch to it. Previously the first agent created required a GUI restart
to select.
Also removes the Remote field from NewAgentDialog — remote belongs to
the session, not the agent.