add debounce to Dropdown #906

Job	Run time
Build Dash Package	3m 9s
Detect Relevant Path Changes	4s
Typing Tests	2m 55s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	0s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	0s
Table Server Tests (Group ${{ matrix.test-group }})	0s
	6m 8s

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