Use a dropdown instead of a text input for overriding comparisons in the component settings

[wooferzfg]

Similar to overriding the timing method, we should show a dropdown that includes the names of all the available comparisons.

We should also decide how to handle the case where a comparison override doesn't match a comparison that's available in the current run. In the original LS, we handled this by disabling the comparison override in this case. However, we still showed the comparison override in the dropdown so that the user wouldn't accidentally disable it in the settings just by visiting the settings.

[CryZe]

I believe so far the idea was to simply indicate that it's a comparison from livesplit-core's side, but still keep it as essentially a textbox, and then simply populate the combobox values in the frontend.

[wooferzfg]

I think that's fine as long as the front end can tell apart a comparison setting value from a text setting value and there's an easy way to get a list of all the comparisons for the current run.