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/project/item/item.cpp

@@ -20,6 +20,8 @@
 
 #include "item.h"
 
+OLIVE_NAMESPACE_ENTER
+
 Item::Item() :
   parent_(nullptr),
   project_(nullptr)
@@ -200,3 +202,5 @@ bool Item::ChildExistsWithNameInternal(const QString &name, Item *folder)
 
   return false;
 }
+
+OLIVE_NAMESPACE_EXIT