Implementing of "Add sort" sub menu in the header of the grid.

[Getty]

This shortcut will appear if you already have a sorting activated and is a way to activate multisort and additional append the specific field to the sorting. This is like the different direction then to click directly on the menu for sorting.

The implementation is pretty straightforward adding a new menu which is only shown when required via a check on beforeMenuShow in the multisort plugin.

This still misses icons for this submenu items.

[coveralls]

Coverage remained the same at 50.95% when pulling 89a80c563a768c8a99a76233dd02d7f1bc8b389e on getty/multisort into 69056f0b8ef1f39382cc9f2e917298976461b46d on master.