Fixed MinGW build by declaring the minimal required Windows version.

Also deleted miscellaneous remaining multimon cruft.
Deleted #include <winuser.h>, because <windows.h> includes it
already.

This should have a nice side effect of preventing us from
accidentally using any Windows API that's too new.
(cherry picked from commit 795c108f2b)

modules/highgui/src/precomp.hpp

@@ -57,6 +57,14 @@
 #include <assert.h>
 
 #if defined WIN32 || defined WINCE
+    #if !defined _WIN32_WINNT
+        #ifdef HAVE_MSMF
+            #define _WIN32_WINNT 0x0600 // Windows Vista
+        #else
+            #define _WIN32_WINNT 0x0500 // Windows 2000
+        #endif
+    #endif
+
     #include <windows.h>
     #undef small
     #undef min