#5127 Implement extra-offset for system positioning

[David Nalesnik]

Implement extra-offset for system positioning

Add the property 'extra-offset to 'line-break-system-details.

It is possible to position systems absolutely using the properties
'X-offset and 'Y-offset of 'line-break-system-details. Placement,
however, is relative to the top left of the usable space, and can
be difficult to gauge without considerable trial and error.

An 'extra-offset property allows easy movement of systems relative
to where they currently are on the page--either through their
default positioning, or as a result of absolute positioning through
'X-offset and/or 'Y-offset.

System separators will be properly positioned. (Thanks go to Thomas
Morley.)

http://codereview.appspot.com/324810043

[David Nalesnik]

Changes based on Harm's review

http://codereview.appspot.com/324810043

[Anonymous]

Passes make, make check and a full make doc.

[Anonymous]

Patch on countdown for May 6th.

(although can you check David K's comments on Rietveld in case you have not already seen them?).

[David Nalesnik]

I checked them. They shouldn't hold back this particular patch.

[Anonymous]

Patch counted down - please push.

[David Nalesnik]

Pushed to staging as commit 24dd6365d3d05f13b9259f547be9b2517851a5d1.