kcms/notifications: modernize Application Settings page

This commit modernizes the design of the page, emulating some of the
recent VDG-directed changes to the Baloo KCM. The changes include:

- Use KCM.AbstractKCM as the root component
- Make it frameless
- Move the search field above everything else, since its scope is
  everything on the page, not just the apps list
- Center the app icon and name and increase the outer margins,
  because I just couldn't find a way to make them look good when
  left-aligned in this new layout
- Add fancy list view header bar
- When the app doesn't support per-event configuration, don't hide
  the list view where that stuff would appear; instead just use it
  as a place to show the placeholder message
- Remove unnecessary full stops from placeholder messages