various small code cleanups and improvements

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

app/task/import/import.cpp

@@ -31,10 +31,10 @@
 #include "panel/project/project.h"
 // End test code
 
+#include "core.h"
 #include "project/item/footage/footage.h"
 #include "task/probe/probe.h"
 #include "task/taskmanager.h"
-#include "undo/undostack.h"
 
 ImportTask::ImportTask(ProjectViewModel *model, Folder *parent, const QStringList &urls) :
   model_(model),
@@ -67,7 +67,7 @@ bool ImportTask::Action()
 bool ImportTask::Epilogue()
 {
   if (command_ != nullptr) {
-    olive::undo_stack.push(command_);
+    Core::instance()->undo_stack()->push(command_);
   }
 
   parent_->UnlockDeletes();

app/task/taskmanager.cpp

@@ -23,7 +23,7 @@
 #include <QDebug>
 #include <QThread>
 
-TaskManager olive::task_manager;
+TaskManager TaskManager::instance_;
 
 TaskManager::TaskManager()
 {
@@ -35,8 +35,13 @@ TaskManager::~TaskManager()
   Clear();
 }
 
+TaskManager *TaskManager::instance()
+{
+  return &instance_;
+}
+
 void TaskManager::AddTask(TaskPtr t)
-{  
+{
   // Connect Task's status signal to the Callback
   connect(t.get(), SIGNAL(StatusChanged(Task::Status)), this, SLOT(TaskCallback(Task::Status)));
 
@@ -125,12 +130,12 @@ TaskManager::AddTaskCommand::AddTaskCommand(TaskPtr t, QUndoCommand *parent) :
 
 void TaskManager::AddTaskCommand::redo()
 {
-  olive::task_manager.AddTask(task_);
+  TaskManager::instance()->AddTask(task_);
 }
 
 void TaskManager::AddTaskCommand::undo()
 {
-  olive::task_manager.DeleteTask(task_.get());
+  TaskManager::instance()->DeleteTask(task_.get());
 
   task_->ResetState();
 }

app/task/taskmanager.h

@@ -70,6 +70,8 @@ public:
    */
   TaskManager& operator=(TaskManager&& other) = delete;
 
+  static TaskManager* instance();
+
   /**
    * @brief Add a new Task
    *
@@ -165,6 +167,11 @@ private:
    */
   int maximum_task_count_;
 
+  /**
+   * @brief TaskManager singleton instance
+   */
+  static TaskManager instance_;
+
 private slots:
   /**
    * @brief Callback when a Task's status changes
@@ -180,8 +187,4 @@ private slots:
 
 };
 
-namespace olive {
-extern TaskManager task_manager;
-}
-
 #endif // TASKMANAGER_H