draw shape/arrow/nodes/paper symbols
This commit removes some more instances where changes in the
canvas affect compositions. Previously the symbols drawn
in shape/arrow/etc items were using the scale from the canvas.
This meant that the appearance of these items in composer
would change depending on canvas zoom if they used symbols
with map unit sizes.
Now they take their map scale from the composition's reference
map. While this has the nice side effect that now map units
can be used in the appearance of these items and they're
guaranteed to match up with the reference map item, the main
intention here is to remove more of the forced links between
compositions (core) and the main canvas (app).
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
- New class QgsComposerObject, which both QgsComposerItem and QgsComposerMultiFrame
derive from. This class contains the framework for data defined composition properties.
- New class QgsComposerUtils, containing helpful static functions previously in QgsComposerItem
- Test suites for data defined settings in QgsComposerObject and functions in QgsComposerUtils
