various small code cleanups and improvements

Largely refactoring work to make the code somewhat nicer to work with.

app/undo/undostack.cpp

@@ -20,9 +20,7 @@
 
 #include "undostack.h"
 
-OliveUndoStack olive::undo_stack;
-
-void OliveUndoStack::pushIfHasChildren(QUndoCommand *command)
+void UndoStack::pushIfHasChildren(QUndoCommand *command)
 {
   if (command->childCount() > 0) {
     push(command);

app/undo/undostack.h

@@ -23,7 +23,7 @@
 
 #include <QUndoStack>
 
-class OliveUndoStack : public QUndoStack {
+class UndoStack : public QUndoStack {
 public:
   /**
    * @brief A wrapper for push() that either pushes if the command has children or deletes if not
@@ -33,11 +33,4 @@ public:
   void pushIfHasChildren(QUndoCommand* command);
 };
 
-namespace olive {
-/**
- * @brief A static undo stack for undoable commands throughout Olive
- */
-extern OliveUndoStack undo_stack;
-}
-
 #endif // UNDOSTACK_H