Add color differentiation for tags groups in overview agenda
Color differentiation improves readability of any data. Currently, activities are represented as blocks of two colors: bright - for those located in future, pale - for those located in past and present. That's okay for the default option, but we can improve it adding combobox for tags group selection. When user selects a tags group (like Priority or Place), activities blocks' colors should change to activity's selected group tag's color. So if High priority is red and Low is green and Priority was selected as a group, all blocks on agenda that refer to activities where priority is High should be colored red, etc.