qt: fix backdrop blur effect not available with KWin Wayland

Currently, the effect is applied before the window is shown. Frankly, there is no restriction from the compositors side (DWM, KWin) regarding when the backdrop blur effect can be activated, and it was a courtesy for the compositors to request the effect even before the window is shown so that they could do the job potentially better.

For us, it does not matter much when the blur is applied. However, with KWin Wayland it does not seem to be possible to use effects before the window is shown. It is not clear if this is a KWin regression, but since it does not matter, the blur can be requested when the window is activated.

Request review @chub.