Decode HTML entities when copying text to clipboard (fixes #1850)

Currently, when using `Copy` in the context menu, the text will be
HTML escaped (e.g. `"` becomes `"`). Add a function to decode
HTML entities when copying to clipboard.

This PR fixes issue #1850.

src/Clipboard.cpp

@@ -5,8 +5,8 @@
 #include "Clipboard.h"
 
 #include <QClipboard>
-#include <QTextDocumentFragment>
 #include <QGuiApplication>
+#include <QTextDocumentFragment>
 
 Clipboard::Clipboard(QObject *parent)
   : QObject(parent)