[partition] Refactor slot for change in action

- Let's not go overboard on lambdas,
 - Needed for others that also want to update the partitioning preview

src/modules/partition/gui/ChoicePage.cpp

@@ -305,14 +305,7 @@ ChoicePage::setupChoices()
     m_rightLayout->setStretchFactor( m_previewAfterFrame, 0 );
 
     connect( this, &ChoicePage::actionChosen,
-             this, [=]
+             this, &ChoicePage::onActionChanged );
-    {
-        Device* currd = selectedDevice();
-        if ( currd )
-        {
-            applyActionChoice( currentChoice() );
-        }
-    } );
 
     CALAMARES_RETRANSLATE(
         updateSwapChoicesTr( m_eraseSwapChoices );
@@ -412,6 +405,17 @@ ChoicePage::continueApplyDeviceChoice()
 }
 
 
+void
+ChoicePage::onActionChanged()
+{
+    Device* currd = selectedDevice();
+    if ( currd )
+    {
+        applyActionChoice( currentChoice() );
+    }
+}
+
+
 void
 ChoicePage::applyActionChoice( ChoicePage::InstallChoice choice )
 {

src/modules/partition/gui/ChoicePage.h

@@ -109,6 +109,9 @@ private slots:
     void onEncryptWidgetStateChanged();
     void onHomeCheckBoxStateChanged();
 
+    /** @brief Calls applyActionChoice() as needed. */
+    void onActionChanged();
+
 private:
     void updateNextEnabled();
     void setupChoices();