qml: do not create csd shadow effect if it is not used

This follows the approach that has been working well in a lot of places: create the item once it is necessary to be used and do not destroy it.

I don't think it is worth to use Loader just to load and unload the effect depending on whether CSD is active, which can be changed while the application is alive. This is because it is not expected that the user switches CSD many times, and the effect is simple enough to be just kept alive instead. Loader is usually used for heavy items.

In any case it does not make sense to create it if the window does not support CSD, or the platform handles the shadows.

Request review @chub.