Avoid flickering of space info on startup
Prior to this commit, when Dolphin was opening in a directory for which the free space information cannot be retrieved, the free space info in the status bar would still briefly be visible before hiding for good.
This commit avoids this flickering by keeping the space info hidden until space info has been successfully retrieved. There is no use showing an empty/wrong space info before that anyway.
I assume the error in the previous code was that it assumed that one could prevent a widget from being shown by overriding QWidget::showEvent(). This does not work because this method is only called to notify QWidgets of their state change.
This commit was primarily written because the brief showing of an empty space info was messing with my WIP automatic tests but this then turned out to be of no importance in the end.