Ability to sort by multiple columns in DataGrid

[knstvk]

FlowUI:

3 new XML attributes:
multiSort (boolean): enable or disable sorting
multiSortOnShiftClickOnly (boolean): enables multiple sorting only when the shift key is pressed
multiSortPriority (enum): the logic for adding new columns to sorting

[KremnevDmitry]

For QA:

How to test

1. FlowUI:

   • Set the multisort attribute for the DataGrid in the XML
   • Open the entity-list view
   • Click on column headers
     Multi sorting should be added
   • Set the multiSortPriority attribute to APPEND for the DataGrid
   • The order in which columns are added to the sort should change
   • Set the multiSortOnShiftClickOnly attribute to true
   • Multi sorting should only be available with shift-click

2. ClassicUI:

   • Open the browser screen
   • Single sorting should be available by clicking on the column header.
   • Multi sorting should only be available with shift-click

[konyashkina]

Tested on

• 2.1-SNAPSHOT
• 1.6-SNAPSHOT