swpagerelsize: add an eWidthPercentRelation member to SwFmtFrmSize

Relative width so far was always relative to margin. This new member can
describe if the relative width should be relative to e.g. page.

Change-Id: Idaed3c1ade8cd49021ed3406abd6f79cf8e742be
swpagerelsize: implement RelativeWidthRelation UNO property for SwXFrame

Which exposes SwFmtFrmSize::eWidthPercentRelation. Do not document yet,
will be done if layout/filters/UI confirms that the data structure is
suitable.

Change-Id: Ic4eac3356452f4bedf9809ff5678e299b7339053
swpagerelsize: take care of Sz.GetWidthPercentRelation() in SwFlyFrm::CalcRel

This allows two type of relative sizes: relative to margin (default,
what we had before) and relative to the entire page.

Change-Id: I1625f072365e418352f3d507764e03484bb812be