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);

app/dialog/export/export.cpp

@@ -204,7 +204,9 @@ ExportDialog::ExportDialog(ViewerOutput *viewer_node, QWidget *parent) :
   // Set defaults
   format_combobox_->setCurrentIndex(kFormatMPEG4);
   video_tab_->width_slider()->SetValue(viewer_node_->video_params().width());
+  video_tab_->width_slider()->SetDefaultValue(viewer_node_->video_params().width());
   video_tab_->height_slider()->SetValue(viewer_node_->video_params().height());
+  video_tab_->height_slider()->SetDefaultValue(viewer_node_->video_params().height());
   video_tab_->set_frame_rate(viewer_node_->video_params().time_base().flipped());
   audio_tab_->set_sample_rate(viewer_node_->audio_params().sample_rate());
   audio_tab_->set_channel_layout(viewer_node_->audio_params().channel_layout());

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++;

app/dialog/footageproperties/streamproperties/videostreamproperties.cpp

@@ -80,6 +80,7 @@ VideoStreamProperties::VideoStreamProperties(ImageStreamPtr stream) :
     imgseq_start_time_ = new IntegerSlider();
     imgseq_start_time_->SetMinimum(0);
     imgseq_start_time_->SetValue(video_stream->start_time());
+    imgseq_start_time_->SetDefaultValue(video_stream->start_time());
     imgseq_layout->addWidget(imgseq_start_time_, imgseq_row, 1);
 
     imgseq_row++;
@@ -89,6 +90,7 @@ VideoStreamProperties::VideoStreamProperties(ImageStreamPtr stream) :
     imgseq_end_time_ = new IntegerSlider();
     imgseq_end_time_->SetMinimum(0);
     imgseq_end_time_->SetValue(video_stream->start_time() + video_stream->duration() - 1);
+    imgseq_end_time_->SetDefaultValue(video_stream->start_time() + video_stream->duration() - 1);
     imgseq_layout->addWidget(imgseq_end_time_, imgseq_row, 1);
 
     video_layout->addWidget(imgseq_group, row, 0, 1, 2);

app/dialog/speedduration/speedduration.cpp

@@ -93,6 +93,7 @@ SpeedDurationDialog::SpeedDurationDialog(const rational& timebase, const QList<C
     speed_slider_ = new FloatSlider();
     speed_slider_->SetMinimum(0);
     speed_slider_->SetDisplayType(FloatSlider::kPercentage);
+    speed_slider_->SetDefaultValue(1);
     speed_layout->addWidget(speed_slider_, row, 1);
 
     if (same_speed) {
@@ -111,6 +112,7 @@ SpeedDurationDialog::SpeedDurationDialog(const rational& timebase, const QList<C
     duration_slider_ = new TimeSlider();
     duration_slider_->SetTimebase(timebase_);
     duration_slider_->SetMinimum(1);
+    duration_slider_->SetDefaultValue(Timecode::time_to_timestamp(clips_.first()->length(), timebase_));
     speed_layout->addWidget(duration_slider_, row, 1);
 
     if (same_duration) {