Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (file-contains-trailing-whitespace)
• Bump debhelper from old 11 to 12. (package-uses-old-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)
• Set upstream metadata fields: Name (from ./configure), Repository. (upstream-metadata-file-is-missing)
• Update standards version to 4.4.1, no changes needed. (out-of-date-standards-version)

This merge proposal was created automatically by the Janitor bot.

You can follow up to this merge proposal as you normally would.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/mousepad/8e08a2bf-1f24-4f1e-bc09-084f71cd6405.

These changes have no impact on the binary debdiff.

You can also view the diffoscope diff (unfiltered).