dead code removal: there is no id=w2ui-panel; only an id=w2ui-popup

(More specifically: there IS a w2ui-panel CLASS (all panels created by the w2layout have that fclass) but there is no w2ui-panel ID. Hence this is all dead code.)
Merge pull request #437 from GerHobbelt/patch-dead-code-removal1

dead code removal: there is no id=w2ui-panel; only an id=w2ui-popup