Implement a (qa-only) quad-tiled viewer.

Implement a (qa-only) quad-tiled viewer.

I.e. we subdivide the document into 4 tiles: one at 100% scaling,
one at 200%, one at 50%, one at 25% -- these are then post-scaled
in gdk) and assembled to show as one document again.

This is specifically a test only widget, primarily to be able to quickly
spot any tile positioning/border-transition issues.

We could theoretically make this widget inherit from the original widget,
however that would mean having to introduce virtual methods etc., which
is not something that we'd want in production -- in the longer run
that widget will hopefully be extended to have proper tile composition etc.,
which would then break this widget too if it were inheriting from there.

Change-Id: Ib880a1614f89724135e753013cf91aec25973e39