[HiDPI] Add interface to QWindow to set the device pixel ratio manually

Why it is needed?
In some cases when AA_EnableHighDpiScaling is enabled, Qt automatically changes the DPR scale for the window according to the screen DPR. The software developer is unable to set it on its own - independently from the Qt.

Use cases:

• override automatically set of DPR,
• allow the application to behave the same way on all platforms,
• allow to set the DPR manually avoiding automatic changes when resolution/screen changes,
• avoid possible auto-dpr bugs,

Usage proposal:

auto w = new QWindow;
...
w->setDevicePixelRatio(2.0);

What happens:

• AA_EnableHighDpiScaling is ignored for this window,
• window is permanently set to DPR of 2.0,