DolphinQt: Replace QStringLiteral with alternatives where applicable

QStringLiterals generate a buffer so that during runtime there's very
little cost to constructing a QString. However, this also means that
duplicated strings cannot be optimized out into a single entry that gets
referenced everywhere, taking up space in the binary.

Rather than use QStringLiteral(""), we can just use QString{} (the
default constructor) to signify the empty string. This gets rid of an
unnecessary string buffer from being created, saving a tiny bit of
space.

While we're at it, we can just use the character overloads of particular
functions when they're available instead of using a QString overload.
The characters in this case are Latin-1 to begin with, so we can just
specify the characters as QLatin1Char instances to use those overloads.
These will automatically convert to QChar if needed, so this is safe.

Source/Core/DolphinQt/Debugger/CodeViewWidget.cpp

@@ -422,7 +422,7 @@ void CodeViewWidget::OnSelectionChanged()
   }
   else if (!styleSheet().isEmpty())
   {
-    setStyleSheet(QStringLiteral(""));
+    setStyleSheet(QString{});
   }
 }