Add gui.tabWidth config

[stefanhaller]

• PR Description

This adds a config gui.tabWidth that can be used to change the tab width from the default of 4 to a different number. It affects everything that's shown in the main view, but is probably mostly relevant for diffs.

When using a pager, the pager needs to be configured separately for the same tab width.

This may often be different for different repos, so it may make sense to put it in a repo-local config file.

In some cases this may still not be flexible enough, e.g. in multi-language projects that use different tab widths for the different file types; but it's better than before and was easy to do...

Addresses #4290.

• 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]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 56c9d5a1	✅ 90.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (56c9d5a)	Report Missing	Report Missing	Report Missing
Head commit (1161619)	53022	45922	86.61%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4291)	20	18	90.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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[jesseduffield]

LGTM