Fixes#15373 - refine current rule not working when symbol widgets
are not used in the style dock
This fixes a behavioural difference when new panels are opened
in a QgsPanelWidget when in docked/undocked mode. When in
docked mode, the newly opened panel will emit panelAccepted
when it is accepted. But for undocked mode, the parent
panel was emitting the panelAccepted signal and so the
connection to update the renderer was never
triggered. Now both docked/undocked panels will always
emit panelAccepted ONLY from the newly opened panel itself.
This also fixes memory leaks as the clean up code was never
run in undocked mode.
I've updated the docs to clarify this behaviour.
Clean up API. Remove QgsPanelWidgetPage - no longer needed
QgsPanelWidgetStack widget manages back button and breadcrumb.
QgsPanelWidget manages state of active widget.
