Added sort by priority and sort by multiple conditions

[alexxandar]

I've added option Priority in "Sort by" menu, also now it's possible to sort items with by several options. This is achieved using the plus sign (+) in the Sortby menu. It works in following way, when you want to sort tasks by one option, you click the option name, and when you want to add another option for sorting, you click the + next to the option name. In that way you add another sorting condition.

This is a resolve for issue: Sort by Priority #91