Try using Qt::Tool for StyleEditorWindow

I think this looks good on macOS and GNOME, but needs to be tested
on other platforms as well.

On macOS, a tool window will stay on top of the main window, but it will
hide when another application is selected.

src/styleeditorwindow.cpp

@@ -9,7 +9,7 @@
  * Initializes the window components and configures the StyleEditorWindow
  */
 StyleEditorWindow::StyleEditorWindow(QWidget *parent)
-    : QDialog(parent),
+    : QDialog(parent, Qt::Tool),
       m_ui(new Ui::StyleEditorWindow),
       m_currentlyClickedButton(Q_NULLPTR),
       m_currentSelectedFontButton(Q_NULLPTR),