Splits the rendering component of annotations out from map
canvas item component.

A new core abstract base class QgsAnnotation handles the
management of the common properties associated with an
annotation, and handles rendering the annotation onto a
QgsRenderContext destination.

Existing annotation classes have been ported to this, and
with the exception of the form annotation moved into core.
Form annotations are dependant on editor widgets and must
remain in GUI.

A new QgsMapCanvasAnnotationItem item class implements
a QgsMapCanvasItem which draws an annotation inside the
canvas, and handles synchronising the position and size
of the canvas item with the QgsAnnotation position/size.

This allows annotations to be safely used in a multi-canvas
environment, with a single QgsAnnotation being displayed
in multiple canvases (even if the canvases have different
extent/crs/etc).

Additionally it allows annotations to be directly rendered
to a map (eg in composer) without going through the
gui based Qt graphics scene framework.

Also removes lots of duplicate code, and adds some basic
unit tests for annotations.