Add support for collapsible panes

[noahp]

PR Description

Enable collapsing the side panels (2,3,4,5) by pressing the given key binding
again after focusing on the side panel. Retains last setting between runs of
lazygit.

Relates to #4600

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 7 duplication

Metric	Results
Complexity	0
Duplication	7

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