Grouping grid

Base ticket for all other Grouping tickets
This ticket handles the headers between the rows when sorting on a group. For each follow up ticket a new group implementation will show a different header.

Expected:

This ticket implements the intended 'look and feel '  and the actual 'support' of the group headers, for re-use in other grouping implementations. 

• Ability to add group headers between items
• Individual items must still be usable (based on concerns by Ronald in comment section).
• When 'show in groups' is enabled, sorting on other columns than then ones we have implemented grouping for should not be enabled.

Note:
Default grouping is not enabled. Default is the 'normal grid'

QA note:

• In order to test this ticket, KW-2347 and KW-1988 will have to be ready as well. Flag this ticket during QA until all tickets can be merged
• merge tickets in this order KW-1988 -> KW-2347 -> KW-2318 -> master

Styling of the headers:

• height: 24px
• color: grey theme color (for basic it is #e6e6e6)
• text color: theme color
• margin left 12px