Add a modal to select a new value for single-valued references #2116
Description
The newly added reference widget only supports setting a reference value through drag'n'drop from the explorer (which is not easily discoverable).
It should provide a "Browse"/"Select" button to open an explorer-like modal where the user can search for and select the new value for the reference.
The reference widget supports both single and multi-valued reference, but we will start here with the simpler case of single-valued references. Multi-valued ones require a different UI (two-pane) and will be handled afterwards.