Make background image toggle more visible

[ShrimpCryptid]

Use Case

Requested by Chantelle! The background image toggle is currently buried in the Viewer settings tab. Can we make it more prominent so users who aren't familiar with the app are more likely to toggle it?

A few areas were suggested for this, either in the top section above the canvas (currently contains just the toggle for keeping range) or in the toolbar.

This is also related to #303, "redesign viewer settings!"

Acceptance Criteria

Please describe how you know this is done

Details

• Do users just want to toggle it on and off from here? Will they need to switch between multiple background images?
• Is selecting the first background image in the dataset okay as a default?
• If TFE was a nucmorph-only app, this toggle might read, "Show original images" and also decrease the object opacity when toggled. Are there ideas from this that we can reuse?
  • What generalizes and what doesn't? (changing object transparency when background images are enabled?)
• Semi-related also to Remove Collapse from SettingsTab. #472, where we could add a visibility toggle for background images

[ShrimpCryptid]

Update:

• Opacity should be filed under the Backdrop image settings
• Opacity should only be applied when backdrops are visible
• Backdrop visibility should now be a new state boolean and synced to the URL
  • This allows the currently selected backdrop to be saved to URL state even if it's not currently visible.