[calamares] Refactor center-placement

- Just move it to its own function where it can have more documentation
 - Tested on multi-screen setups

SEE #1293

src/calamares/CalamaresApplication.cpp

@@ -343,6 +343,41 @@ CalamaresApplication::initModuleManager()
     m_moduleManager->init();
 }
 
+/** @brief centers the widget @p w on (a) screen
+ * 
+ * This tries to duplicate the (deprecated) qApp->desktop()->availableGeometry()
+ * placement by iterating over screens and putting Calamares in the first
+ * one where it fits; this is *generally* the primary screen.
+ * 
+ * With debugging, it would look something like this (2 screens attached,
+ * primary at +1080+240 because I have a very strange X setup). Before
+ * being mapped, the Calamares window is at +0+0 but does have a size.
+ * The first screen's geometry includes the offset from the origin in
+ * screen coordinates.
+ * 
+ *  Proposed window size: 1024 520 
+ *  Window QRect(0,0 1024x520) 
+ *  Screen QRect(1080,240 2560x1440) 
+ *  Moving QPoint(1848,700) 
+ *  Screen QRect(0,0 1080x1920) 
+ *
+ */
+static void
+centerWindowOnScreen( QWidget* w )
+{
+    QList< QScreen* > screens = qApp->screens();
+    QPoint windowCenter = w->rect().center();
+
+    for ( const auto* screen : screens )
+    {
+        QPoint screenCenter = screen->availableGeometry().center();
+        if ( !itFits && ( screenCenter.x() >= windowCenter.x() ) && ( screenCenter.y() >= windowCenter.y() ) )
+        {
+            w->move( screenCenter - windowCenter );
+            break;
+        }
+    }
+}
 
 void
 CalamaresApplication::initView()
@@ -360,17 +395,7 @@ CalamaresApplication::initView()
 
     if ( Calamares::Branding::instance() && Calamares::Branding::instance()->windowPlacementCentered() )
     {
-        QList< QScreen* > screens = qApp->screens();
-        QPoint windowCenter = m_mainwindow->rect().center();
-        for ( const auto* screen : screens )
-        {
-            QPoint screenCenter = screen->availableGeometry().center();
-            if ( ( screenCenter.x() >= windowCenter.x() ) && ( screenCenter.y() >= windowCenter.y() ) )
-            {
-                m_mainwindow->move( screenCenter - windowCenter );
-                break;
-            }
-        }
+        centerWindowOnScreen( m_mainwindow );
     }
     cDebug() << "STARTUP: CalamaresWindow created; loadModules started";
 }