Fix window title not being shown by embedded KPart
DocumentViewPrivate::updateCaption() does not change the caption when mCurrent is false (as introduced by commit a7819f6e). It is correctly set when using the Gwenview application, but it is never set by the part and so the window title is never shown.
Since there is no image selection or slideshow when using the part, it is enough to simply call setCurrent() in openUrl().