Make Windows-1250 encoding consistent with other

In sake of consistency with other Windows CP encodings:
* print_name is expanded to "Windows Central European (CP 1250)";
* B_MS_WINDOWS_1250_CONVERSION id looks like should be added into UTF8.h;
* mime_name set to NULL as other windows codepages have. That prevents
  at least from duplicating too much 1250's in the Terminal, Mail and
  StyledEdit encodings menus.

headers/os/support/UTF8.h

@@ -39,7 +39,8 @@ enum {
 	B_ISO15_CONVERSION,
 	B_BIG5_CONVERSION,				// Chinese Big5
 	B_GBK_CONVERSION,				// Chinese GB18030
-	B_UTF16_CONVERSION				// Unicode UTF-16
+	B_UTF16_CONVERSION,				// Unicode UTF-16
+	B_MS_WINDOWS_1250_CONVERSION	// Windows Central European Codepage
 };