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/codec/h264section.cpp

@@ -121,6 +121,7 @@ H264CRFSection::H264CRFSection(QWidget *parent) :
   crf_input->SetMinimum(kMinimumCRF);
   crf_input->SetMaximum(kMaximumCRF);
   crf_input->SetValue(kDefaultCRF);
+  crf_input->SetDefaultValue(kDefaultCRF);
   layout->addWidget(crf_input);
 
   connect(crf_slider_, &QSlider::valueChanged, crf_input, &IntegerSlider::SetValue);