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.

resources/qml/MessageView.qml

@@ -477,7 +477,7 @@ Item {
                     text: qsTr("&Copy")
                     visible: messageContextMenuC.text
 
-                    onTriggered: Clipboard.text = messageContextMenuC.text
+                    onTriggered: Clipboard.setHtmlText(messageContextMenuC.text)
                 }
             }
             Component {
@@ -679,7 +679,7 @@ Item {
                     text: qsTr("&Copy")
                     visible: replyContextMenuC.text
 
-                    onTriggered: Clipboard.text = replyContextMenuC.text
+                    onTriggered: Clipboard.setHtmlText(replyContextMenuC.text)
                 }
             }
             Component {

src/Clipboard.cpp

@@ -6,6 +6,7 @@
 
 #include <QClipboard>
 #include <QGuiApplication>
+#include <QTextDocumentFragment>
 
 Clipboard::Clipboard(QObject *parent)
   : QObject(parent)
@@ -19,6 +20,12 @@ Clipboard::setText(QString text)
     QGuiApplication::clipboard()->setText(text);
 }
 
+void
+Clipboard::setHtmlText(QString text)
+{
+    setText(QTextDocumentFragment::fromHtml(text).toPlainText());
+}
+
 QString
 Clipboard::text() const
 {

src/Clipboard.h

@@ -21,6 +21,7 @@ public:
 
     QString text() const;
     void setText(QString text_);
+    Q_INVOKABLE void setHtmlText(QString text_);
 signals:
     void textChanged();
 };