Add build support for Windows Phone 8.1

Tweak qmake, add mkspecs for emulator and device, adjust the
manifest template for WP8.1, and add missing icons.

Change-Id: I7a6405fa85297ae4cc8522015274e65fb7a315a6
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Support Windows Phone 8.1 in WinRT QPA

- Unsupported code paths for WP8.0 are avoided, and new APIs are used
  where appropriate (virtual keyboard)
- DirectWrite fonts are loaded on WP8.1
- Platform dialogs are used on WP8.1

Change-Id: I721006ac943ad4e248f0f1590ce247a03e40fbc0
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
ANGLE WinRT: Create swap chain using physical resolution

ANGLE has been creating the framebuffer in logical pixels instead of
physical pixels, which leads to unexpected results and side effects like
smudged anti-aliased text. This fixes the issue by multiplying the DIP
resolution by the scale factor, making the framebuffer match the physical
pixel resolution of the screen.

Change-Id: I3594995ce8e18a31b47e27165f72bc6a391b97b6
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
WinRT: Support High-DPI

Previously, the backing store and default framebuffer were created with
the logical screen resolution (in device-independent pixels), not the
the physical screen resolution. This lead to blurry text on high-DPI
devices. This change fixes this by creating those at full size, and
setting the device pixel ratio appropriately. Windows are still reported
in device-independent pixels, but text and images are now rendered
sharply for Qt Quick applications.

As QPainter does not support non-integer scaling, the backing store is
still drawn in DIPs and scaled by OpenGL.

Task-number: QTBUG-38464
Change-Id: I7377d4c734126825d670b8ebb65fd0dd1ef705f2
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>