RunAction: Fix string conversion from bytestring, correct codestyle

Summary:

• Fix action execution when url wrongly converted to string as bytestring. This resulted in not working actions (loading b"b"url"")
• Make codestyle more pythonic
• Remove not required imports
• Configue dialog:
  • Set label weight to bold in QtDesigner (UI file)
  • Make listview scroll per pixel

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D21718