Make mouse usage/highlight optional

[woolsweater]

I personally find the mouse highlight a bit distracting* and would like to be able to disable it. I suggest that it be made optional with a new customization -- selectrum-allow-mouse-selection or something like that. It would be t by default, so as not to break expectations.

It seems to be a simple change, and I have a branch locally that does what I want. Is there any appetite for this? If so, I can submit a PR.

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*It's mostly caused by my use of mini-frame and putting the child frame in the middle of the main frame -- so that the mouse often happens to be in the child frame's bounds.