Add terms stats to the terms panel.

his pr allow user to compute values in term panel using the following modes :

count
total
min
max
mean
When you select other mode than count :

A field "Value field" appear so you can choose on which field the computation is performed.
The "Exclude Terms(s)" field is hidden, because terms stats is not
The sort field will contain more options : total, reverse_total, min, reverse_min, max, reverse_max, mean, reverse_mean
Please let me know if something should be updated.