new identify method: feature selection

![image](https://cloud.githubusercontent.com/assets/127259/3798733/a3934f22-1be7-11e4-84b6-40cb478f40d1.png)

This adds a new identify method to be able to identify a single method and thus always open the desired feature form.

I did not find a nice way to combine *layer selection* with *feature selection* due to the fact that a menu item can not be clicked (only actions can). Otherwise, mixing the two might be a problems for some since it would need one more level to reach the old functionnality (open identify dock with all results of a single layer). This is why I added a new method.
Anyway, one might wants to remove the old behavior by this one?