Polish the Icons example.

- Remove unneeded member variables.
- Use new connection syntax in createActions()
  to assemble the menu there, removing the createMenus()
  function.
- Use a QButtonGroup for the sizes with the style metrics as id.
- Streamline code, rename variables for clarity.
- Introduce static functions returning the list of states/modes and
  their names in display order to IconPreviewArea, removing the
  duplicates.
- Split actions to open sample icons provided from source directory
  or standard pictures location to make sample icons more prominently
  visible.
- Check and display @2x images and add tooltips showing device pixel
  ratio and actual size.
- HighDPI: Add groupbox displaying screen name and device pixel
  ratio, connect to screen changed signal and update icons panel
  accordingly. Add check box for toggling Qt::AA_UseHighDpiPixmaps.
- Adjust documentation

Task-number: QTBUG-18680
Task-number: QTBUG-46615
Change-Id: Ice96e9898f168ef2a30e9f46cb260ed57ae015f0
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>