Apply additional zoom to the webview based on screen DPI.

Also change `defaultZoomLevel()`'s `const int &` return type to just `int`.

Test Plan:
On Linux:
In KDE, forced DPI to 96 and 120. Restarting the app shows initial
webview scaled to 1x and 1.25x respectively.
In Gnome 3, use Tweaks to set font scaling to 1.25, restart the app.

src/libs/browser/webview.cpp

@@ -39,9 +39,11 @@
 using namespace Zeal::Browser;
 
 WebView::WebView(QWidget *parent)
-    : QWebView(parent)
+    : QWebView(parent), m_zoomLevel(0)
 {
     page()->setNetworkAccessManager(Core::Application::instance()->networkManager());
+    // Applies any DPI based scaling.
+    setZoomLevel(defaultZoomLevel());
 }
 
 int WebView::zoomLevel() const
@@ -60,7 +62,10 @@ void WebView::setZoomLevel(int level)
 
     m_zoomLevel = level;
 
-    setZoomFactor(availableZoomLevels().at(level) / 100.0);
+    // Scale the webview relative to the DPI of the screen.
+    qreal dpiZoomFactor = logicalDpiY() / 96.0;
+
+    setZoomFactor(availableZoomLevels().at(level) / 100.0 * dpiZoomFactor);
     emit zoomLevelChanged();
 }
 
@@ -72,7 +77,7 @@ const QVector<int> &WebView::availableZoomLevels()
     return zoomLevels;
 }
 
-const int &WebView::defaultZoomLevel()
+int WebView::defaultZoomLevel()
 {
     static const int level = availableZoomLevels().indexOf(100);
     return level;

src/libs/browser/webview.h

@@ -41,7 +41,7 @@ class WebView : public QWebView
     void setZoomLevel(int level);
 
     static const QVector<int> &availableZoomLevels();
-    static const int &defaultZoomLevel();
+    static int defaultZoomLevel();
 
 public slots:
     void zoomIn();
@@ -65,7 +65,7 @@ public slots:
 
     QMenu *m_contextMenu = nullptr;
     QUrl m_clickedLink;
-    int m_zoomLevel = defaultZoomLevel();
+    int m_zoomLevel;
 };
 
 } // namespace Browser