Fetch news from remmina.org optional

This patch add a compile flag, -DWITH_NEWS that can be set to OFF to disable the changelog fetching from remmina.org.

The UI elements are updated to show a placeholder text and disclaimers.

The "Fetch news from remmina.org" switch is disabled and cannot be changed when WITH_NEWS=OFF

This should fix https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=961501

Tests

• News are disabled at compile time.
  • Clean installation test.
  • Existing installation.
  • +7 days tests, the news widget should not start.
  • Remmina preferences are write protected.
• News are disabled at compile time.
  • Clean installation test.
  • Existing installation.
  • +7 days tests, the news widget should show up if there's a new version.
  • Remmina preferences are write protected.