[Gui] preferences: add button to switch gradient colors

- when finding a suitable background color gradient, it is annoying that there is no button to switch the begin/end color of the gradient. With the button one can quickly see the the changes and find a suitable gradient much quicker

src/Gui/DlgSettingsViewColor.cpp

@@ -22,6 +22,10 @@
 
 #include "PreCompiled.h"
 
+#ifndef _PreComp_
+# include <QPushButton>
+#endif
+
 #include "DlgSettingsViewColor.h"
 #include "ui_DlgSettingsViewColor.h"
 
@@ -41,6 +45,7 @@ DlgSettingsViewColor::DlgSettingsViewColor(QWidget* parent)
     ui->setupUi(this);
     ui->HighlightColor->setEnabled(ui->checkBoxPreselection->isChecked());
     ui->SelectionColor->setEnabled(ui->checkBoxSelection->isChecked());
+    connect(ui->SwitchGradientColors, &QPushButton::pressed, this, &DlgSettingsViewColor::onSwitchGradientColorsPressed);
 }
 
 /**
@@ -98,5 +103,12 @@ void DlgSettingsViewColor::changeEvent(QEvent *e)
     }
 }
 
+void DlgSettingsViewColor::onSwitchGradientColorsPressed()
+{
+    QColor tempColor = ui->backgroundColorFrom->color();
+    ui->backgroundColorFrom->setColor(ui->backgroundColorTo->color());
+    ui->backgroundColorTo->setColor(tempColor);
+}
+
 #include "moc_DlgSettingsViewColor.cpp"