Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)
• Bump debhelper from old 11 to 13. (package-uses-old-debhelper-compat-version)
• Set upstream metadata fields: Bug-Database, Bug-Submit, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Update standards version to 4.6.0, no changes needed. (out-of-date-standards-version)
• Remove field Section on binary package libgxr-0.15-0 that duplicates source. (installable-field-mirrors-source)

This merge proposal was created by the Janitor bot, and it will automatically rebase or close this proposal as appropriate when the target branch changes. Any comments you leave here will be read by the Janitor's maintainers.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/gxr/ceb7bdf1-1e31-41b4-932e-e3406c3eb3ff.

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/lib/debug/.build-id/2e/deb0699c2b13a6e48239961616fa6ad8143415.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/78/9789cbda17ace49b6ad2e53ea943e5e214ca5a.debug
-rw-r--r--  root/root   /usr/lib/debug/.dwz/x86_64-linux-gnu/libgxr-0.15-0.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/11/da02d5d7c749837b34fb4d34ac6d430a7aacee.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/17/65424eb9bc765360f6ff0c07336e86ce5a1f3c.debug

No differences were encountered between the control files of package **libgxr-0.15-0**

Control files of package libgxr-0.15-0-dbgsym: lines which differ (wdiff format)

• Build-Ids: 11da02d5d7c749837b34fb4d34ac6d430a7aacee 1765424eb9bc765360f6ff0c07336e86ce5a1f3c 2edeb0699c2b13a6e48239961616fa6ad8143415 789789cbda17ace49b6ad2e53ea943e5e214ca5a

No differences were encountered between the control files of package **libgxr-dev**

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