Qt: small header changes / fixes

- ensure header inclusion guard is named after the header file
- add missing comments at the end of some inclusion guards
- add a small Qt5 compatibility fix in macdockiconhandler.h

src/qt/bitcoinamountfield.h

@@ -1,5 +1,5 @@
-#ifndef BITCOINFIELD_H
-#define BITCOINFIELD_H
+#ifndef BITCOINAMOUNTFIELD_H
+#define BITCOINAMOUNTFIELD_H
 
 #include <QWidget>
 
@@ -57,4 +57,4 @@ private slots:
 };
 
 
-#endif // BITCOINFIELD_H
+#endif // BITCOINAMOUNTFIELD_H