FFE? some blending fixes & composition feature requests

Once again, I'll let someone else make the call on these given the timing. Included (with python bindings and tests!) are some features and fixes which would be great for 2.0:

- feature blend modes. this restores the original behaviour of vector layer blend modes so that the blending is applied between the features on the same layer. (as opposed to the layer blend mode, which is still present as is the method used to apply that layer to other layers below it). I've had a number of requests for this, and it makes it possible again to create maps like http://sphotos-f.ak.fbcdn.net/hphotos-ak-prn1/163413_479288597199_8388607_n.jpg 

- layer opacity for vector layers. see #7692. 

- blend modes for composer map grids (#7777)

- font color choice for grid annotations (#7789) - a noticeable omission