Move actual generation of the blurred image to its own helper script

As the "blur" operation returns a promise it needs to be ensured that
the elements will not be used after the component was destroyed.

Similarly the background should be restored after getting the "raw"
size of the image element to prevent an empty background to be shown
while the new background is being generated (although in practice this
will not happen due to "blur" still generating the image in the UI
thread, but it will be needed once moved to a worker)

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>