This commit simplifies the button code for Elisa in the following ways:
- Use ToolButtons rather than flat Buttons for controls than want to
  look like ToolButtons. This is possible now that our QQC2 style shows
  ToolButton focus correctly.
- Remove hardcoded custom button sizes and let them be their natural
  size so they adjust with the scale factor and font size properly.
- Make HeaderFooterToolbar a bit smaller to fit the new noew-smaller
  ToolButtons.
- Redo the custom contentItem for the sort button to work properly with
  standardized buttons.