Make featuresgrid content selectable

For some reason the default behaviour of ExtJS' Grids is to disable cell content selecting. This PR reenables it so that users may copy-paste those content.

Taken from http://ahlearns.wordpress.com/2011/10/27/ext-js-make-grid-text-selectable/ itself refering to http://www.sencha.com/learn/grid-faq/