Use a thread lock for Clipboard.qimage()

Prevents this: user paste image (image provider retrieves the clipboard
image to show it as a thumbnail),
and immediatly presses enter (retrieves the image to convert it and send
to Python); the two calls would run at the same time instead of one
waiting for the other to set the cache.

src/clipboard.h

@@ -13,6 +13,8 @@
 #include <QIODevice>
 #include <QImage>
 #include <QMimeData>
+#include <QMutex>
+#include <QMutexLocker>
 #include <QObject>
 
 
@@ -47,6 +49,8 @@ public:
     }
 
     QImage *qimage() {
+        QMutexLocker locker(&(this->imageRetrievalLock));
+
         if (this->cachedImage.isNull())
             this->cachedImage = this->clipboard->image();
 
@@ -100,6 +104,7 @@ signals:
 private:
     QClipboard *clipboard = QGuiApplication::clipboard();
     QImage cachedImage    = QImage();
+    QMutex imageRetrievalLock;
 
     void mainClipboardChanged() {
         this->contentChanged();