SVG with constraints

I miss a lot in this kind of programs connectors
between two objects that represent a restriction. A
restriction can be of type:

A) position. A fixed distance or a percent with respect
the main (grouped objects) object (when size is
modified). Latter means that when the main object is
streched (or shrinked) the distance between the two
connected objects is a percent of the "streched distance"

B) color. When a color of 1 connector-side is modified,
the color of the other connector-side is also modified
(no necesarily 1 to 1, i.e., one color is function of
the other color).

C) Properties in general. As in the case of the color,
the same can be applied to stroke, fill-color etc etc

To show the use of this connectors-restriction, let's
imagine we define a "window" with a title-bar. When
strecthing the window, we desire to have the same
distance between the border of the window and the
title-bar (look whatever window in your desktop).

As another example, we can think about Open File
dialog. When streching the dialog, the frame that shows
the file should be stretched keeping a fix distance
with the dialog borders.

Also should be possible to exclude elements from the
strech.

The idea is to use the .svg file to paint widgets...
;). When I was a younger (16 y.o.) I implemented this
kind of restrictions via formulaes, p.eg., title.X =
window.X + 10. title.text.height << window.height*0.01
where << means that title height is incremented in 1%
respect to the window.height streching.