Fix remaining Qt6 build warnings (narrowing, nodiscard, qHash)

Clears the 9 non-deprecation warnings from the Qt6 build:
- qHash(QColor): hash rgba() (unambiguous QRgb) instead of name(), and
  use the size_t seed signature on Qt6 (guarded for Qt5). Fixes the
  ambiguous-overload warning in terminalstripmodel.h.
- Two qsizetype->int narrowings in brace-init: explicit static_cast<int>
  (elementscene.cpp, terminalstrip.cpp).
- main.cpp: keep the QtConcurrent::run QFuture in a [[maybe_unused]]
  variable (nodiscard).
- qetapp.cpp: guard the stylesheet load on QFile::open() succeeding
  (nodiscard) instead of ignoring the result.

sources/editor/elementscene.cpp

@@ -748,7 +748,7 @@ void ElementScene::addItems(QVector<QGraphicsItem *> items)
  */
 void ElementScene::removeItems(QVector<QGraphicsItem *> items)
 {
-	const int previous_selected_count{selectedItems().size()};
+	const int previous_selected_count = static_cast<int>(selectedItems().size());
 
 		//block signal to avoid multiple emit of selection changed,
 		//we emit this signal only once at the end of this function.