Fix some issues reported by lintian

Fix some issues reported by lintian

• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• Move transitional package libgdk-pixbuf2.0-dev to oldlibs/optional per policy 4.0.1. (transitional-package-not-oldlibs-optional)
• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact.
• Update standards version to 4.6.2, no changes needed. (out-of-date-standards-version)
• Remove field Section on binary package libgdk-pixbuf2.0-dev that duplicates source. (installable-field-mirrors-source)
• Avoid explicitly specifying -Wl,--as-needed linker flag. (debian-rules-uses-as-needed-linker-flag)

These changes have no impact on the binary debdiff.

You can also view the diffoscope diff (unfiltered).

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Build and test logs for this branch can be found at https://janitor.debian.net/run/e96dae57-c1e6-4732-a65c-a9b7e09cd842.