winrt: enable hardware buttons for Windows 10

Previously the hardware and camera button handler were guarded inside a
Q_OS_WINPHONE which does not apply to Windows 10.

Instead use WINAPI_PARTITION_FAMILY like on other places, this covers
Windows Phone 8.1 as well as Windows 10.

To find windows.phone.ui.input.h at build time the Mobile Extension
directory needs to be added to the include paths inside qmake. On
runtime we need to check whether we have hardware buttons or not. In
case they exist, register the handlers, otherwise skip registration.
Skipping also helps to keep WACK succeeding.

Task-number: QTBUG-50427
Change-Id: Ibeae15dbde12553cebd2b73b1a40b754c014f426
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>