Add ringbuffer sink

Ringbuffer sink keeps user-given number of most recent log messages in
memory and returns them upon request (using the ringbuffer_sink::last
method). This can be useful for e.g. remote debugging of a running app.

"#include" <spdlog/.*> instead of "spdlog/.*"

The meaning of using quotes to #include is implementation defined, so it
may or not may be what we want. At least POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html)
says: "headers whose names are enclosed in double-quotes ( "" ) shall be
searched for first in the directory of the file with the #include line",
so not what we want since "spdlog" ends up twice in the path.

Correctly guard SetHandleInformation API call

`__cplusplus_winrt` only detected C++/CX (which can be used without compiling for UWP, SetHandleInformation would be available in those cases), and did not detect native UWP C++. This patch fixes that by using the WINAPI_FAMILY_PARTITION macro in the Windows SDK headers in the same way those headers remove SetHandleInformation in UWP builds.