Previously only some annotations had (incomplete) support for
attaching to a particular vector layer and synchronising their
visibility with the layer's visibility.
This handling has all been moved up to QgsAnnotation, so that
all annotation types can be attached to layers.
This will allow selective annotation visibility based on the
visible layers of a particular canvas, eg in multi-canvas
environments.
Additionally:
- show the attached layer in the annotation properties
dialog, and allow it to be cleared to always show the
annotation
- allow attaching annotations to non-vector layers
- add unit tests for visibility
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.
