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/core.h

@@ -52,12 +52,62 @@ class Core : public QObject
 {
   Q_OBJECT
 public:
+  class CoreParams
+  {
+  public:
+    CoreParams();
+
+    enum RunMode {
+      kRunNormal,
+      kHeadlessExport,
+      kHeadlessPreCache
+    };
+
+    bool fullscreen() const
+    {
+      return run_fullscreen_;
+    }
+
+    void set_fullscreen(bool e)
+    {
+      run_fullscreen_ = e;
+    }
+
+    RunMode run_mode() const
+    {
+      return mode_;
+    }
+
+    void set_run_mode(RunMode m)
+    {
+      mode_ = m;
+    }
+
+    const QString startup_project() const
+    {
+      return startup_project_;
+    }
+
+    void set_startup_project(const QString& p)
+    {
+      startup_project_ = p;
+    }
+
+  private:
+    RunMode mode_;
+
+    QString startup_project_;
+
+    bool run_fullscreen_;
+
+  };
+
   /**
    * @brief Core Constructor
    *
    * Currently empty
    */
-  Core();
+  Core(const CoreParams& params);
 
   /**
    * @brief Core object accessible from anywhere in the code
@@ -66,7 +116,10 @@ public:
    */
   static Core* instance();
 
-  int execute(QCoreApplication *a);
+  const CoreParams& core_params() const
+  {
+    return core_params_;
+  }
 
   /**
    * @brief Start Olive Core
@@ -407,14 +460,6 @@ private:
    */
   MainWindow* main_window_;
 
-  /**
-   * @brief Internal startup project object
-   *
-   * If the user specifies a project file on the command line, the command line parser in Start() will write the
-   * project URL here to be loaded once Olive has finished initializing.
-   */
-  QString startup_project_;
-
   /**
    * @brief List of currently open projects
    */
@@ -456,14 +501,14 @@ private:
   QStringList recent_projects_;
 
   /**
-   * @brief Internal variable for whether the GUI is active
+   * @brief Parameters set up in main() determining how the program should run
    */
-  bool gui_active_;
+  CoreParams core_params_;
 
   /**
    * @brief Static singleton core instance
    */
-  static Core instance_;
+  static Core* instance_;
 
 private slots:
   void SaveAutorecovery();