General usability improvements

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The current version possesses several usability flaws:

• Play button in steps panel tends to be obscured due to width, even though playing back search traces is an extremely basic use case. Fix this by moving playback controls onto a separate line.
• Edit layer panel has no confirm button, so users have no feedback/assurance that their changes went through.
• When choosing a search trace, users tend to click Type: Trace from the dropdown and expect to be prompted to choose a file. However, they must also click on Choose Trace below that dropdown to do so.
• Layers panel does not scroll
• Clicking on a layer does not do anything, even though users expect it to open the Edit Layer panel.

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In 28d4b97, we fixed:

• [x] Playback controls in a separate line
• [x] Layers panel does not scroll
• [x] Clicking a layer opens the Edit Layer panel
• [x] Choosing layer type is less ambiguous through a redesign:

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• [ ] Edit layer panel has no confirm button, so users have no feedback/assurance that their changes went through.

Should test if the edit layer panel is unconfusing enough to not need a confirm button, since now it's a popover

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General quality stuff

• [ ] Consistent language, terminology and copywriting (sentence case everything)
• [ ] Localisation (put all strings in a JSON file)