Use 'const' and QStringLiteral where possible

Co-authored-by: ronso0 <ronso0@mixxx.org>

src/widget/wbasicpushbutton.cpp

@@ -71,16 +71,14 @@ void WBasicPushButton::paintEvent(QPaintEvent* /*unused*/) {
 QString WBasicPushButton::buildToolTip() const {
     // Show the button label as a tooltip when the label text
     // is partially hidden due to the button's size
-
-    QString txtToolTip = toolTip();
-
+    const QString txtToolTip = toolTip();
     if (!txtToolTip.isEmpty()) {
         return txtToolTip;
     }
 
-    QString txtLabel = text();
-    QSize currentSize = size();
-    QSize fullSize = sizeHint();
+    const QString txtLabel = text();
+    const QSize currentSize = size();
+    const QSize fullSize = sizeHint();
 
     if ((currentSize.height() < fullSize.height() ||
                 currentSize.width() < fullSize.width()) &&
@@ -96,7 +94,7 @@ QString WBasicPushButton::buildToolTip() const {
 bool WBasicPushButton::event(QEvent* e) {
     switch (e->type()) {
     case QEvent::ToolTip: {
-        QString toolTipToShow = buildToolTip();
+        const QString toolTipToShow = buildToolTip();
         if (!toolTipToShow.isEmpty()) {
             QToolTip::showText(static_cast<QHelpEvent*>(e)->globalPos(),
                     toolTipToShow,

src/widget/wcolorpicker.cpp

@@ -51,7 +51,7 @@ WColorGridButton::WColorGridButton(const mixxx::RgbColor::optional_t& color,
         // Set the background color of the button.
         // This can't be overridden in skin stylesheets.
         setStyleSheet(
-                QString("QPushButton { background-color: %1; }")
+                QStringLiteral("QPushButton { background-color: %1; }")
                         .arg(mixxx::RgbColor::toQString(color.value())));
         setToolTip(mixxx::RgbColor::toQString(color.value()));
     } else {