Implement app.dock.downloadFinished

[cndouglas]

Make the Downloads folder dock icon "bounce" when a download is complete (macOS-only).

Resolves #1667

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

1. Navigate to https://brave.com/ .
2. Click Get Brave and then click Save.
3. As the download progress bar approaches 100%, look at the Downloads folder dock icon.
4. Make sure the dock icon "bounces" when the download finishes.

[cndouglas]

Question: should the code be placed in electronDownloadItem.js or app/filtering.js?

[bbondy]

It's good there. Thanks, works great.

[alexwykoff]

I wasn't able to replicate in 0.15.307, did this get reverted?

[luixxiul]

d3ff9d9

[luixxiul]

so it seems like it's not.

[cndouglas]

It's working for me with the latest master (6a9a017).

[cndouglas]

Also works for me with 0.15.307 on macOS 10.12.x.

[alexwykoff]

we're currently building 0.15.3xx releases off of dev-channel, so master may be ahead 😄

[cndouglas]

@alexwykoff Good to know. I can confirm this feature is working with the version just released (0.15.310, 6b5e4e2).