Fix some issues reported by lintian

Fix some issues reported by lintian

• debian/copyright: use spaces rather than tabs to start continuation lines. (tab-in-license-text)
• Bump debhelper from old 11 to 12. (package-uses-old-debhelper-compat-version)
• Update standards version to 4.5.0, no changes needed. (out-of-date-standards-version)

This merge proposal was created automatically by the Janitor bot.

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/xfce4-places-plugin/cc511973-3025-4fe0-8ac8-76c5e3438708.

These changes have no impact on the binary debdiff.

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