Objects selection on sketches

It is not possible to selects objects if the object on the stack is a sketch

```python
s = Sketch().rect(10,5)
b = Workplane().placeSketch(s).wires()
print(b.ctx.pendingWires) # return []
```
I needed this because in order to loft several sketchs I need to push their wires onto the context otherwise they arent usable anymore after workplane offsettings.

I think this should also be an issue, I feel it should be possible to do :

``` python
s = Sketch().rect(10,5)
b = Workplane().placeSketch(s).workplane(10).placeSketch(s).loft()
```

It would probably be a huge refactor but I think while they arent used for any 3D operations, Sketchs should be available from the context. Once they are used they should be tagged as so and so they wouldn't be reused by other 3D ops later on the stack.