Improve and modernize UI of Addons sheet

Various improvements:
- Put the bottom buttons in a footer so they can't scroll out of view
- Rename "Discard" button to "Reset" since "Discard" implies closing the
  sheet
- Move Apply and Reset buttons over to the right side, as is typical
- Disable rather than hide the Apply and Reset buttons
- Remove the icons since they add no information and were all the same
- Make subtext less opaque rather than italic, as is our style (we can't
  simply port to `Kirigami.BasicListItem` yet because it can't support
  showing a checkbox to the left of the icon)

BUG: 425827
FIXED-IN: 5.20