various: encapsulate all code in the olive namespace to avoid name collisions

Large-scale code cleanup. Also adds a license to the top of all files that were
missing it.

app/dialog/about/about.h

@@ -23,6 +23,10 @@
 
 #include <QDialog>
 
+#include "common/define.h"
+
+OLIVE_NAMESPACE_ENTER
+
 /**
  * @brief The AboutDialog class
  *
@@ -45,4 +49,6 @@ public:
   explicit AboutDialog(QWidget *parent = nullptr);
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif // ABOUTDIALOG_H