Polish rich text example.

- Introduce Qt 5 signals & slot syntax.
- Use mime types in the file dialogs.
- Streamline the code creating the actions.
- Introduce QCommandLineParser.
- Query the available size when determining
  the initial size instead of using hard-coded values
  for High-DPI screens.

Change-Id: Ifc84a41ed55a4a674b6eafdb6120ac42441405b6
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>