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/Config/Mapping/MappingButton.cpp

@@ -32,8 +32,8 @@ constexpr int SLIDER_TICK_COUNT = 100;
 // Escape ampersands and remove ticks
 static QString ToDisplayString(QString&& string)
 {
-  return string.replace(QStringLiteral("&"), QStringLiteral("&&"))
-      .replace(QStringLiteral("`"), QStringLiteral(""));
+  return string.replace(QLatin1Char{'&'}, QStringLiteral("&&"))
+      .replace(QLatin1Char{'`'}, QString{});
 }
 
 bool MappingButton::IsInput() const