Colour by secondary structure conservation

Add colour by secondary structure consensus as a colouring option in the colour menu. This should allow user to apply a bleaching effect proportional to the secondary structure consensus percentage score on the existing colouring. Moreover, this option should be available in the "Group Colour" (Selection -> Edit Group -> Group Colour) menu item in the pop up menu that appears when the user right clicks on a group. The option should allow user to edit the colouring for the groups.