Prevent ugly select bar to appear when touching slideshow

While testing the slideshow I try to switch to the next pic by touching the screen (my laptop has a touchscreen, yay) and an ugly blue selection appear because I happened to drag the cursor a bit.

Could happen on tablets as well.

This PR prevents a select highlight to appear when dragging/touchsliding inside the slideshow.

@DeepDiver1975 @owncloud/designers @icewind1991