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/folder/folder.cpp

@@ -25,9 +25,7 @@
 #include "project/item/sequence/sequence.h"
 #include "ui/icons/icons.h"
 
-Folder::Folder()
-{
-}
+OLIVE_NAMESPACE_ENTER
 
 Item::Type Folder::type() const
 {
@@ -91,3 +89,5 @@ void Folder::Save(QXmlStreamWriter *writer) const
 
   writer->writeEndElement(); // folder
 }
+
+OLIVE_NAMESPACE_EXIT

app/project/item/folder/folder.h

@@ -25,6 +25,8 @@
 #include "project/item/footage/footage.h"
 #include "project/item/item.h"
 
+OLIVE_NAMESPACE_ENTER
+
 /**
  * @brief The Folder class representing a directory in a project structure
  *
@@ -34,7 +36,7 @@
 class Folder : public Item
 {
 public:
-  Folder();
+  Folder() = default;
 
   virtual Type type() const override;
 
@@ -50,4 +52,6 @@ private:
 
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif // FOLDER_H