WinUIBackend: Set window minimum size

[stackotter]

WinUI doesn't expose a way to set the minimum size of a window, so we don't currently do it. When adding support for file dialogs we had to get access to the underlying HWND of the window, which was rather complicated. Now that we can get the HWNDs of arbitrary windows, we can probably implement minimum window sizing by reaching past WinUI. There is some commented out code in WinUIBackend's createWindow method from a previous attempt at resolving this issue. I can't remember what the issue was with it; it probably just didn't work. Regardless, it should provide a good starting point.