part: stop changing the window title when the toolbar gets edited

KParts::ReadOnlyPart::guiActivateEvent(), which emits `setWindowCaption`
with url().toDisplayString() as argument, gets called when the toolbar
gets changed. We don't want this behavior, so we override
guiActivateEvent() with an empty implementation that does nothing.

BUG: 357660
FIXED-IN: 16.12.0