Provide controls to manipulate z-order explicitly

[Peter Clifton]

gschem's drawing order is based upon file order in the schematic. Certain schematics may be making use of this fact (which we try diligently to preserve when making code changes).

It would be useful to expose controls for the user to control the Z-order of drawn objects, along the lines of:

Send to back
Move backwards
Move forwards
Bring to front

Inkscape uses this terminology / menu structure:

Object->Raise
Object->Lower
Object->Raise to top
Object->Lower to bottom