dcc: refactor dropdown #388

Job	Run time
Detect Relevant Path Changes	11s
Build Dash Package	3m 11s
Typing Tests	3m 2s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	0s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	0s
Table Server Tests (Group ${{ matrix.test-group }})	0s
	6m 24s

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