Dropdown multiple select

[tilen1976]

Is your feature request related to a problem? Please describe.
You can only select one item from Dropdown list, there is a need for users to select multiple (for instance in Datadoc)

Describe the solution you'd like
User can select multiple options, chosen options are visible as selected (all with a x to remove them one by one). selectedItem is replaced by selectedItems.

Describe why this is useful for SSB
Requested by Datadoc, but should be useful elsewhere in SSB as well

Additional context