The scrolling concepts seem quite obscure and firmly tied to concepts like
"pages" and "lines". For touch devices one would want to be able to request
scrolling simply in *pixels*.

It is the scrollbar objects (in vcl and toolkit, and yes, UNO is also involved
over in toolkit) that all the action goes through so they must exist even if
they should not be displayed.

(Both on Android and iOS we should presumably use these system-specific
ephemeral scroll indicator thingies that show up only during scrolling but
otherwise fade away. Hmm, probably the VCL (or toolkit?) scrollbar code should
use platform-specific code to display those then.)

Change-Id: I1f82648561e41ae25f2427fd0fe60249c927a9a1