Fix macOS Qt 5.9+ close boxes do not show on document mode tab bar tabs

Setting any stylesheet makes the default close button disappear.
Setting a custom close button temporarily works around issue.

See: https://bugreports.qt.io/browse/QTBUG-61092
     https://bugreports.qt.io/browse/QTBUG-61742

Author: Larry Shaffer

images/images.qrc

@@ -365,6 +365,8 @@
         <file>themes/default/mIconColorWheel.svg</file>
         <file>themes/default/mIconConnect.png</file>
         <file>themes/default/mIconClose.svg</file>
+        <file>themes/default/mIconCloseTab.svg</file>
+        <file>themes/default/mIconCloseTabHover.svg</file>
         <file>themes/default/mIconCritical.png</file>
         <file>themes/default/mIconDataDefine.svg</file>
         <file>themes/default/mIconDataDefineError.svg</file>

images/themes/default/mIconCloseTab.svg

@@ -104,6 +104,21 @@ void QgisAppStyleSheet::buildStyleSheet( const QMap<QString, QVariant> &opts )
 
   ss += QStringLiteral( "* { font: %1pt \"%2\"} " ).arg( fontSize, fontFamily );
 
+#if QT_VERSION >= 0x050900
+  // Fix for macOS Qt 5.9+, where close boxes do not show on document mode tab bar tabs
+  // See: https://bugreports.qt.io/browse/QTBUG-61092
+  //      https://bugreports.qt.io/browse/QTBUG-61742
+  // Setting any stylesheet makes the default close button disappear.
+  // Specifically setting a custom close button temporarily works around issue.
+  // TODO: Remove when regression is fixed (Qt 5.9.3 or 5.10?); though hard to tell,
+  //       since we are overriding the default close button image now.
+  if ( mMacStyle )
+  {
+    ss += QLatin1String( "QTabBar::close-button{ image: url(:/images/themes/default/mIconCloseTab.svg); }" );
+    ss += QLatin1String( "QTabBar::close-button:hover{ image: url(:/images/themes/default/mIconCloseTabHover.svg); }" );
+  }
+#endif
+
   // QGroupBox and QgsCollapsibleGroupBox, mostly for Ubuntu and Mac
   bool gbxCustom = opts.value( QStringLiteral( "groupBoxCustom" ) ).toBool();
   QgsDebugMsg( QString( "groupBoxCustom: %1" ).arg( gbxCustom ) );