PowerfulWhenNeeded: Various changes to layout, structure and text

This MR introduces four (squashable!) commits:

• PowerfulWhenNeeded: Use page actions for external app links
  • This uses Kirigami page actions for links (e.g. "Open KDE Connect in System Settings"), which is more consistent with Kirigami patterns. The text is made shorter "Open Settings..." because the context is clear based on the page, and to ensure it fits with room to spare in the minimum width.
• qml: Move PlasmaFeatureButton to pages subfolder
  • This moves PlasmaFeatureButton as it is a type used only by the features page. This is consistent with how network has a subfolder for PlasmaNM.
• PowerfulWhenNeeded: Adjust GetNewStuff description
  • This uses italic text instead of square brackets in the description.
• PowerfulWhenNeeded: Remove "See More Features" buttons
  • This removes the button from each feature layer.

Before	After