Fix some issues reported by lintian

Fix some issues reported by lintian

• Update lintian override info format in d/transgui.lintian-overrides on line 3, 5. (mismatched-override)
• Remove executable bit from desktop files. (executable-desktop-file)
• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• Set upstream metadata fields: Bug-Database, Bug-Submit. (upstream-metadata-missing-bug-tracking)
• Use canonical URL in Vcs-Browser, Vcs-Git. (vcs-field-not-canonical)
• Remove obsolete field Name from debian/upstream/metadata (already present in machine-readable debian/copyright).
• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

Debdiff

These changes affect the binary packages:

File lists identical (after any substitutions)

Control files: lines which differ (wdiff format)

• Depends: libatk1.0-0 (>= 1.12.4), libc6 (>= 2.2.5), 2.34), libcairo2 (>= 1.2.4), libgdk-pixbuf-2.0-0 (>= 2.22.0), libglib2.0-0 (>= 2.12.0), libgtk2.0-0 (>= 2.24.0), libpango-1.0-0 (>= 1.18.0), libx11-6

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/transgui/07f5303e-0637-40c5-9f1b-5993dda20bb0.