kcms/baloo: modernize style

- Use ScrollViewKCM as the base element and move the non-view UI
  elements to the view header, so the KCM can't ever be in a situation
  where it has two scrollbars
- Put global-scope "pause/resume indexing" action in the header
- Move status text into the relevant part of the controls layout,where
  its positioning won't seem so random
- Make file path in the status text label selectable
- Use a FormLayout for the main controls
- Flatten list item UI elements and adopt alternating background style
  so they're more visually connected with their text
- Move footer actions to a fancy custom Kirigami-style ListView header
  to avoid the "two stacked rows of buttons" effect. If we like this UI
  element, we can make a generic component out of it and use it in other
  KCMs as well
- Tweak wordings

BUG: 469902
FIXED-IN: 6.0