win32: Add SystemMediaTransportControls support

Implements #25258

This adds a module win_smtc which implements SystemMediaTransportControls integration for Windows, and a config option smtc to toggle this (enabled by default).

Implementation notes

This uses C++/WinRT to consume the WinRT APIs needed. As partial mingw-w64 support (recent GCC and Clang, requires C++20) has recently been added to upstream cppwinrt, it is now functional enough to handle the simple APIs of SystemMediaTransportControls. A copy of cppwinrt headers is downloaded in contrib.

Implemented functions

• Play / pause / stop / prev / next buttons (tested on Windows 10 with native UI and ModernFlyouts)
• Fast forward / rewind buttons which are supposed to jump using "medium-jump-size" (untested as nothing seems to use it)
• Auto repeat toggle (tested on Windows 10 with ModernFlyouts)
• Shuffle toggle (tested on Windows 10 with ModernFlyouts)
• Timeline display and seeking (tested on Windows 10 with ModernFlyouts)
• Media metadata display including album artwork (tested on Windows 10 with native UI and ModernFlyouts)

Unimplemented functions

• Channel up/down buttons
• Playback rate setting
• Record button