Provides all necessary additional data about the area shown to user in
model mode. The popups in Luwrain are the areas of a special type
which main distinguish is that their appearance initiates new instance
of the message loop.
The popups can be shown as a single method invocation but since that
freezes processing of input and environment events this feature is
implemented with launching new temporary copy of event loop
procedure. So the key attribute is a flag which signals that new event
loop can be stopped. If the popup object is prepared its real start
should be done with Luwrain.popup() method.