copy/paste: try to convert geometries

this add a convertToLayerGeometry method in QgsVectorLayer: it tries to transfrom the given geometry to the layer geometry type.

It handles all the possible combinations: (multi)point / (multi)line  / (multi)polygon.

I also added a isMultiPart method to QgsVectorLayer.

Now, paste action will only paste in the layer if the geometry conversion is successful.

This follows the discussion in #1098.
@jef-n, is this a new feature?
If yes, would it be possible to accept #1098 for 2.2 (it would be very very helpful for our job...)?