footageviewer: allows users to drag video or audio only

Updates drag mime data to include a variable determining which streams are
enabled and which ones aren't (a binary qint64 where the 0s are disabled
and the 1s are enabled) rather than relying solely on the stream->enabled
values which can't be modified as part of the drag process.

app/widget/playbackcontrols/dragbutton.cpp

@@ -0,0 +1,12 @@
+#include "dragbutton.h"
+
+DragButton::DragButton(QWidget *parent) :
+  QPushButton(parent)
+{
+}
+
+void DragButton::mousePressEvent(QMouseEvent *event) {
+  QPushButton::mousePressEvent(event);
+
+  emit MousePressed();
+}