Meta: Add a script check the presence of "#pragma once" in header files

.. and make travis run it.

I renamed check-license-headers.sh to check-style.sh and expanded it so
that it now also checks for the presence of "#pragma once" in .h files.

It also checks the presence of a (single) blank line above and below the
"#pragma once" line.

I also added "#pragma once" to all the files that need it: even the ones
we are not check.
I also added/removed blank lines in order to make the script not fail.

I also ran clang-format on the files I modified.

DevTools/HackStudio/Debugger/VariablesModel.h

@@ -25,6 +25,7 @@
  */
 
 #pragma once
+
 #include "Debugger.h"
 #include <AK/NonnullOwnPtr.h>
 #include <LibGUI/Model.h>