allow choosing destination when using context menu

[seansfkelley]

Browser extensions can't programmatically open their own popups, so assuming picking arbitrary directories is the goal, there's only two interactions possible here:

• User selects menu item, is presented with an overlay (or similar) in the page itself that allows them to configure stuff and eventually sends a message to the NAS on its own. This requires injecting a content script on every page and maintaining a UI in vanilla js, which I am not keen to do.
• Users selects menu item, is somehow notified to continue in the popup. This could be a notification or something simple in-page. The UX is a bit disconnected, but it should be a lot easier to maintain.

The suggestion from #128 (comment) is to have a configurable but static list of destinations. This would also interact closely with #181, #217 and perhaps #169, which is appealing.

[panosl]

When we right click a magnet link, can't the "Download with DownloadStation" go a level deeper with the Predefined Locations? That would be a first step, between nothing an totally dynamic option. like:

Download with DownloadStations -> Software
                               -> Kernel
                               -> Linux Distros