cache: reimplemented auto-caching

Implemented a smarter auto-cache that's context sensitive. Caching will
automatically pause when playback begins or values are being changed and resume
when inactive.

app/widget/taskview/taskview.cpp

@@ -47,6 +47,7 @@ void TaskView::AddTask(Task *t)
 {
   // Create TaskViewItem (UI representation of a Task) and connect it
   TaskViewItem* item = new TaskViewItem(t);
+  connect(item, &TaskViewItem::TaskCancelled, this, &TaskView::TaskCancelled);
   items_.insert(t, item);
   layout_->insertWidget(layout_->count()-1, item);
 }

app/widget/taskview/taskview.h

@@ -42,6 +42,9 @@ class TaskView : public QScrollArea
 public:
   TaskView(QWidget* parent);
 
+signals:
+  void TaskCancelled(Task* t);
+
 public slots:
   /**
    * @brief Creates a TaskViewItem, connects it to a Task, and adds it to this widget

app/widget/taskview/taskviewitem.cpp

@@ -61,12 +61,13 @@ TaskViewItem::TaskViewItem(Task* task, QWidget *parent) :
 
   // Connect to the task
   connect(task_, &Task::ProgressChanged, this, &TaskViewItem::UpdateProgress);
-  connect(cancel_btn_, &QPushButton::clicked, task_, &Task::Cancel, Qt::DirectConnection);
+  connect(cancel_btn_, &QPushButton::clicked, this, [this] { emit TaskCancelled(task_); });
 }
 
 void TaskViewItem::Failed()
 {
-  task_status_lbl_->setText(task_->GetError());
+  task_status_lbl_->setStyleSheet("color: red");
+  task_status_lbl_->setText(tr("Error: %1").arg(task_->GetError()));
 }
 
 void TaskViewItem::UpdateProgress(double d)

app/widget/taskview/taskviewitem.h

@@ -47,6 +47,9 @@ public:
 
   void Failed();
 
+signals:
+  void TaskCancelled(Task* t);
+
 private:
   QLabel* task_name_lbl_;
   QProgressBar* progress_bar_;