[calamares] Allow get/set of panel-sides

- Add access to the panel-sides membe of the view manager, and
  calculate which sides are populated by panels (if any).
- Pass the calculated panel-sides to the view manager before it
  starts adding viewpages, so they get consistent margins.

src/calamares/CalamaresWindow.cpp

@@ -237,11 +237,13 @@ CalamaresWindow::getQmlNavigation( QWidget* parent )
 }
 #else
 // Bogus to keep the linker happy
-QWidget * CalamaresWindow::getQmlSidebar(QWidget* , int )
+QWidget*
+CalamaresWindow::getQmlSidebar( QWidget*, int )
 {
     return nullptr;
 }
-QWidget * CalamaresWindow::getQmlNavigation(QWidget* )
+QWidget*
+CalamaresWindow::getQmlNavigation( QWidget* )
 {
     return nullptr;
 }
@@ -401,6 +403,14 @@ CalamaresWindow::CalamaresWindow( QWidget* parent )
     insertIf( mainLayout, PanelSide::Right, navigation, branding->navigationSide() );
     insertIf( mainLayout, PanelSide::Right, sideBox, branding->sidebarSide() );
 
+    // layout->count() returns number of things in it; above we have put
+    // at **least** the central widget, which comes from the view manager,
+    // both vertically and horizontally -- so if there's a panel along
+    // either axis, the count in that axis will be > 1.
+    m_viewManager->setPanelSides(
+        ( contentsLayout->count() > 1 ? Qt::Orientations( Qt::Horizontal ) : Qt::Orientations() )
+        | ( mainLayout->count() > 1 ? Qt::Orientations( Qt::Vertical ) : Qt::Orientations() ) );
+
     CalamaresUtils::unmarginLayout( mainLayout );
     CalamaresUtils::unmarginLayout( contentsLayout );
     baseWidget->setLayout( mainLayout );

src/libcalamaresui/ViewManager.h

@@ -55,7 +55,7 @@ class UIDLLEXPORT ViewManager : public QAbstractListModel
     Q_PROPERTY( bool quitVisible READ quitVisible NOTIFY quitVisibleChanged FINAL )
 
     ///@brief Sides on which the ViewManager has side-panels
-    Q_PROPERTY( Qt::Orientations panelSides MEMBER m_panelSides )
+    Q_PROPERTY( Qt::Orientations panelSides READ panelSides WRITE setPanelSides MEMBER m_panelSides )
 
 public:
     /**
@@ -111,6 +111,9 @@ public:
      */
     bool confirmCancelInstallation();
 
+    Qt::Orientations panelSides() const { return m_panelSides; }
+    void setPanelSides( Qt::Orientations panelSides ) { m_panelSides = panelSides; }
+
 public Q_SLOTS:
     /**
      * @brief next moves forward to the next page of the current ViewStep (if any),