Add skeleton qrenderdoc (just a qt template) & include in linux build

* Building qrenderdoc on windows will be supported just because that's
  likely where I will be developing it mostly, but for the foreseeable
  future it won't be built by default. If it ever gets good enough to
  replace the .NET UI, then we can look at it again.

qrenderdoc/main.cpp

@@ -0,0 +1,15 @@
+#include "mainwindow.h"
+#include <QApplication>
+
+#include "renderdoc_replay.h"
+
+int main(int argc, char *argv[])
+{
+    RENDERDOC_LogText("QRenderDoc initialising.");
+
+    QApplication a(argc, argv);
+    MainWindow w;
+    w.show();
+
+    return a.exec();
+}