This is the start of a dedicated callout shape.
It uses draw:type to identify callouts, same as LO/OO.
Unfortunatwly it abuses odf, but if we shall be compatible
with LO/OO I see no other option.

Roundtrips via LO works both ways and LO's ui also works with
callout produced with stage.

TODO:

• Shearing and rotation
• Needs new tool for resize/move of 'bubble' part

Known problem:
Rounding (?) problem when calculating position/size offsets,
but this will be changed anyway when shearing/rotation is implemented.