Add clang-format file

Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.

Applications/Taskbar/WindowIdentifier.h

@@ -18,6 +18,7 @@ public:
     {
         return m_client_id == other.m_client_id && m_window_id == other.m_window_id;
     }
+
 private:
     int m_client_id { -1 };
     int m_window_id { -1 };