Implement Alt Click to default on sliders

The following sliders now have Alt Click functionality:
 - Nodes (What it's initialy set too or 0 if not)
 - Export Dialog Dimensions~ (Defaults to Sequence width/height)
 - Export Compression Settings~
    - Advanced (0)
 - Speed/Duration
    - Speed (100%)
    - Duration (clip length)
 - Stream Properties Image sequence
    - Start Index (1)
    - End Index (sequence length)

Sliders with no default value set ignore an Alt
Click.

Added SliderBase::SetDefaultValue() which sets the
default value of a slider.

Added ValueReset() to SliderBase signals which is
called if a slider is Alt Clicked on. Only updates
if default_value_ is not Null.

app/widget/slider/sliderbase.cpp

@@ -23,6 +23,7 @@
 #include <QDebug>
 #include <QEvent>
 #include <QMessageBox>
+#include <QGuiApplication>
 
 OLIVE_NAMESPACE_ENTER
 
@@ -49,6 +50,7 @@ SliderBase::SliderBase(Mode mode, QWidget *parent) :
   connect(label_, SIGNAL(dragged(int)), this, SLOT(LabelDragged(int)));
   connect(label_, SIGNAL(drag_stop()), this, SLOT(LabelClicked()));
   connect(label_, SIGNAL(focused()), this, SLOT(LabelClicked()));
+  connect(label_, SIGNAL(ResetResult()), this, SLOT(ValueReset()));
   connect(editor_, SIGNAL(Confirmed()), this, SLOT(LineEditConfirmed()));
   connect(editor_, SIGNAL(Cancelled()), this, SLOT(LineEditCancelled()));
 
@@ -137,6 +139,11 @@ void SliderBase::SetValue(const QVariant &v)
   UpdateLabel(value_);
 }
 
+void SliderBase::SetDefaultValue(const QVariant &v) 
+{ 
+  default_value_ = ClampValue(v);
+}
+
 void SliderBase::SetMinimumInternal(const QVariant &v)
 {
   min_value_ = v;
@@ -241,7 +248,6 @@ void SliderBase::LabelClicked()
     emit ValueChanged(value_);
   } else {
     // This was a simple click
-
     // Load label's text into editor
     editor_->setText(ValueToString(value_));
 
@@ -327,4 +333,12 @@ void SliderBase::LineEditCancelled()
   label_->blockSignals(false);
 }
 
+void SliderBase::ValueReset() 
+{
+  if (!default_value_.isNull()) {
+    SetValue(default_value_);
+    emit ValueChanged(value_);
+  }
+}
+
 OLIVE_NAMESPACE_EXIT