QStyleHelper: Base DPI-calculation on QScreen.

QStyleHelper: Base DPI-calculation on QScreen.

- Use qt_defaultDpiX() to obtain the resolution, which
  obtains it from QScreen. This implies that for X11,
  which previously used a hardcoded default of 96 DPI,
  the real resolution will be used (typically 75).

- Since many tests (layouts, graphicsview) contain
  test data for 96 DPI, add an attribute to
  QCoreApplication making it possible to set the
  resolution to 96 DPI for testing.

Change-Id: I77c8233a96b0d75de07406f58d48886a89c3de06
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>