Support Buffered / Framed labels and callout labels

[timlinux]

User 'ttestt' (Nikolay Zatsenko) submitted a patch to me to partially support these features. I am attaching a modified version of this patch to here for record of the enhancement, though I would suggest that these improvements could be better directed to the new labelling infrastructure. Either way, some refinement is needed before being able to incorporate these changes into trunk since the patch has code style issues and api breakage (though the latter is minimised now with my changes to his original patch).

I like the general idea of Nikolay's patch and it would be great to see something like this supported in QGIS in the future.

Tim

[timlinux]

Nikolay's buffers patch with modifications by Tim