Make the connections bounding points assignable

[Bug Importer]

Currently (as of 0.45) the only point to connect the connection lines to an
objects is the center. That makes the use of InkScape for creating the
technical schemes (like hydraulic, pneumatic and electric) very hard,
because the symbols on these schemes aren`t just the boxes or circles.

For this feature to be implemented, user must be able to specify the point
anywhere on the object, to where the connection line is bound and remains
connected whenever it moved. This is the main trigger.

Later, the more can be made to make the connections avoid crossing with the
objects and lines and the style of it`s routing, ability to make a
crossing/bypassing points with other connections lines, etc.
Well,i guess even the former will help a lot of people with schemes for
their`s diploma`s, thesis and other stuff. Like me =)

Thanks!