main/core: changed code so that headless would run truly headless with QCoreApplication instead of QApplication

QApplication is designed for GUI/widget programs and thus fails to launch
if no window system is available. For a true headless mode, we must swap
out with QCoreApplication. However, what complicates this is that
QCommandLineParser - the class we use to determine whether we should run
in GUI mode or not - requires an active application instance to work.
This creates an unfortunate catch-22 that ultimately ended in writing a
custom command line parser.

Now we can correctly swap out with QCoreApplication, however this has
exposed other issues regarding functions that unnecessarily rely on QWidget
functions. So they'll need to be sorted out eventually.

app/common/CMakeLists.txt

@@ -21,6 +21,8 @@ set(OLIVE_SOURCES
   common/cancelableobject.h
   common/channellayout.h
   common/clamp.h
+  common/commandlineparser.h
+  common/commandlineparser.cpp
   common/crashhandler.h
   common/crashhandler.cpp
   common/crashpadinterface.cpp