Implement single method capture request

The core now has a method requestCapture, which receives a
CaptureRequest object with all the needed information.
This reduces code duplication in future features.

src/utils/screengrabber.cpp

@@ -17,6 +17,7 @@
 
 #include "screengrabber.h"
 #include "src/utils/filenamehandler.h"
+#include "src/utils/systemnotification.h"
 #include <QPixmap>
 #include <QScreen>
 #include <QGuiApplication>
@@ -65,6 +66,9 @@ QPixmap ScreenGrabber::grabEntireDesktop(bool &ok) {
             ok = false;
             break;
         }
+        if (!ok) {
+            SystemNotification().sendMessage(tr("Unable to capture screen"));
+        }
         return res;
     }
 #endif