Change Cell Titles based on Tag Variables

__Proposal:__
Capability for cell title to change when a tag variable is set. Could be any variable but use case could see tags being the most useful (ex: toggling between devices and view their names in the cell header).

__Current behavior:__
In order to see what the data in the cell addresses in a dashboard, need to refer to the top where a variable is selected.

__Desired behavior:__
Define variable in a cell title and have it change dynamically based off selection.

__Alternatives considered:__
None

__Use case:__
Improves dashboard experience with variables.


No files, tests, or entry points are named. Start by locating dashboard cell title rendering and variable handling, then clarify which variables are supported and verify that a selected variable changes the cell title dynamically.

Written by the indexing model from the issue text.