winrt: Clean up platform theme

- Drop "Platform" from class name for consistency
- Move styleHint() into theme class for better code reuse
- Use ComPtr and result checking
- Remove obsolete WinRT ifdefs

Change-Id: Id26bd75739a9665f61ee2e70eb3fb36747b75410
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
winrt: Introduce theme palette

Use the colors returned by WinRT's UISettings class to construct the
system palette.

Change-Id: Id0d99c7b2e3df209de0755ee72a0d2bda67fda8c
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
Reviewed-by: Jochen Seemann <seemann.jochen@gmail.com>
winrt: Clean up platform message dialog helper

- Remove "Platform" from class name for consistency
- Use ComPtr and HRESULT checking where missing
- Use UICommand ID class to reduce callback complexity
- Use dialog completed callback to fix failed repeated dialog opening

These changes have been tested with the QtQuick.Dialogs message dialog
example, and all features appear to be working. Note that the WinRT
dialog supports a maximum of three buttons, though, and a warning is
printed if this number is exceeded.

Similarly to Android, the native hooks can be now be disabled by
using qputenv("QT_USE_WINRT_NATIVE_DIALOGS", "0").

Task-number: QTBUG-38115
Task-number: QTBUG-39868
Change-Id: I9943c7c11bd640790db68219cefdca1a680e96ec
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>