Investigate the possibility of conditionally enabling UWP API's

[geluk]

The immediate cause for this is #72 (Clipboard history items are not deleted).
This happens because we don't have any support for the clipboard history API, since it's Windows 10-only.

I looked into this a little more, and I found that there is actually some good documentation now on how this can be done.
The clipboard history API is UWP-only, while pass-winmenu also supports Windows 8 and will still run on Windows 7 as well. I'd rather not lose that, but it looks like there may actually be a way to use this API and still target multiple Windows versions.

• https://docs.microsoft.com/en-us/windows/apps/desktop/modernize/desktop-to-uwp-enhance#modify-a-net-project-to-use-windows-runtime-apis
• The relevant API is this: https://docs.microsoft.com/en-us/uwp/api/windows.applicationmodel.datatransfer.clipboard.deleteitemfromhistory?view=winrt-19041.

I'm going to experiment with this a bit to see if this can be made to work on pass-winmenu.