[FEATURE] Redesigned interactive gradient editor

Uses new gradient editor widget and color controls for easier
manipulation of gradients. Also adds an interactive plot
for modifying gradient stops via the color HSV or RGB values.

Fix #8383

Sponsored by North Road

python/gui/symbology-ng/qgsvectorgradientcolorrampv2dialog.sip

@@ -11,20 +11,9 @@ class QgsVectorGradientColorRampV2Dialog : QDialog
     void setColor1( const QColor& color );
     void setColor2( const QColor& color );
 
-    void toggledStops( bool on );
-    void addStop();
-    void removeStop();
-
-    void stopDoubleClicked( QTreeWidgetItem* item, int column );
-    void setItemStopColor( const QColor& newColor );
-
   protected slots:
     void on_cboType_currentIndexChanged( int index );
     void on_btnInformation_pressed();
 
   protected:
-
-    void updateStops();
-    void updatePreview();
-    void setStopColor( QTreeWidgetItem* item, const QColor& color );
 };