Fix slow visual feedback on screenshot service

This is re-proposing MR !211 (closed) by Brion Vibber, applying some review comments and rebasing.

Add a "screenshot-taken" signal from the screenshot service's internal C implementation, and use that to trigger the camera flash visual effect and the click sound, allowing them to run in parallel with the PNG compression instead of waiting until the file is complete to start.

This significantly improves perceived latency on high res setups such as 4K, 5K, or dual 4K screens.

Fixes #512 (closed)

Depends on gnome-screenshot!39 (merged), gnome-settings-daemon!223 (merged)

This also moves some strings from g-s-d over to the shell.