Improve buttons of tails-screen-locker

• Use title capitalization as of GNOME Style Guide.
  • Put Cancel button on the left by convention.

Feature Branch: bugfix/15369-screen-locker-buttons

Attachments

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Parent Task: #5684 (closed)

Related issues

• Blocks #10970 (closed)

Original created by @sajolida on 15369 (Redmine)