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/mainwindow.h

@@ -37,6 +37,8 @@
 #include "panel/pixelsampler/pixelsamplerpanel.h"
 #include "project/project.h"
 
+OLIVE_NAMESPACE_ENTER
+
 /**
  * @brief Olive's main window responsible for docking widgets and the main menu bar.
  */
@@ -86,4 +88,6 @@ private slots:
 
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif