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/dialog/export/exportadvancedvideodialog.cpp

@@ -19,6 +19,7 @@ ExportAdvancedVideoDialog::ExportAdvancedVideoDialog(QWidget *parent) :
 
   thread_slider_ = new IntegerSlider();
   thread_slider_->SetMinimum(0);
+  thread_slider_->SetDefaultValue(0);
   layout->addWidget(thread_slider_, row, 1);
 
   row++;