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/window/mainwindow/mainmenu.h

@@ -27,6 +27,8 @@
 #include "dialog/actionsearch/actionsearch.h"
 #include "widget/menu/menu.h"
 
+OLIVE_NAMESPACE_ENTER
+
 /**
  * @brief Olive's main menubar attached to its main window.
  *
@@ -243,4 +245,6 @@ private:
 
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif // MAINMENU_H