Extract AbstractSlideFragment for slide preview updating.

Currently this code is duplicated in some classes, and is missing
for other classes (e.g. the pointer doesn't get updated if the slide
is changed from a different device/on the host slideshow).

Related fdo#81049

Change-Id: I9407cbb233481718d61159d9618ee14e4fe47d4e
fdo#81049 Use AbstractSlideFragment to update pointer-fragment.

This means that the preview shown in laserpointer mode is updated
if the current slide is changed externally (volume buttons, other
remote, or host computer).

Change-Id: Ifad1e2e70abedfa3a4756cf17ea2c3b48f793b6a