Add setting to pre-fetch when window opens

[JamesNZL]

See: Flow-Launcher/Flow.Launcher#2216

Can add a new setting to allow users to opt to perform the pre-fetch when the Flow window opens, rather than waiting for the plugin to trigger with the tgl keyword.

Would effectively eliminate the loading bar currently seen if cache has expired and TogglTrack#RequestResults() has to do all the fetching before returning the results.