[torconnect] Reduce width of location dropdown to ~240px

The dropdown selector on the location settings screen appears overly-long atm:

I'm guessing that's because it's been sized to match its longest option?

Alternatively, ~240px would work for all but ~15 options, which would get truncated when selected. ~264px would work for all but ~7. None of these are countries or regions included in our censorship map and are less likely to be selected.

Can we try ~240px initially please? See the default font dropdown in about:preferences#general for example of the same truncation behavior.