Update Simple Browser example

- Accept downloads and add a downloads list.
- Fix toolbar icons being pixelated on hidpi screens by
  - enabling attribute AA_UseHighDpiPixmaps, and
  - replacing the 22x22 icons with 32x32 versions.
- Move favicon selection to WebView to reduce duplication.
- Replace UrlLineEdit with a standard QLineEdit using a QAction for the favicon
  and setClearButtonEnabled(true) for the clear button.
- Fix bug where the "File -> New Tab" action would create background tabs
  because the QAction::triggered(bool) signal was connected to the
  TabWidget::createTab(bool) slot with the bool argument having a completely
  different meaning between the two.
- Make the toolbar unmovable. Nobody wants to move the toolbar.
- Add tooltips to toolbar buttons.
- Add tooltips to the tab bar (page titles).
- Stop adding icons to menu items only to disable them right after.

Task-number: QTBUG-60655
Change-Id: I10cc0fa82dbf39281bbdbbf9ef901e1b26402f80
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>