Gui: Ability to target new file buttons fia QSS.

This adds ability to target New File buttons via `#CreateNewRow >
QPushButton` selector.

src/Gui/Stylesheets/Dark theme.qss

@@ -1197,6 +1197,11 @@ QPushButton {
   min-width: 80px;
 }
 
+#CreateNewRow > QPushButton {
+  /* Reset min width to default */
+  min-width: -1;
+}
+
 QPushButton:disabled {
   background-color: #444444;
   color: #adadad;

src/Gui/Stylesheets/Light theme.qss

@@ -1123,6 +1123,11 @@ QPushButton {
   min-width: 80px;
 }
 
+#CreateNewRow > QPushButton {
+  /* Reset min width to default */
+  min-width: -1;
+}
+
 QPushButton:disabled {
   background-color: #d8d8d8;
   color: #adadad;

src/Mod/Start/Gui/StartView.cpp

@@ -147,8 +147,18 @@ StartView::StartView(QWidget* parent)
 
     _newFileLabel = gsl::owner<QLabel*>(new QLabel());
     layout->addWidget(_newFileLabel);
+
+    auto createNewRow = gsl::owner<QWidget*>(new QWidget);
     auto flowLayout = gsl::owner<FlowLayout*>(new FlowLayout);
-    layout->addLayout(flowLayout);
+
+    // reset margins of layout to provide consistent spacing
+    flowLayout->setContentsMargins({});
+
+    // this allows new file widgets to be targeted via QSS
+    createNewRow->setObjectName(QStringLiteral("CreateNewRow"));
+    createNewRow->setLayout(flowLayout);
+
+    layout->addWidget(createNewRow);
     configureNewFileButtons(flowLayout);
 
     _recentFilesLabel = gsl::owner<QLabel*>(new QLabel());