Add a tooltip to the memory dropdown ("data source" / "call tree summary strategy")

[mstange]

For the options like "Deallocated memory" + "Deallocation sites", it's not easy to find out what they mean.

We have a tooltip though! It's on the <option>. But you only see the tooltip while the <select> menu is open.

I think we should add the description of the selected option to the <select> itself too, so that you get a tooltip when the menu is closed.

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