chore: prefer QStringLiteral (#1284)

Further reading:
* https://forum.qt.io/topic/78540/qstringliteral-vs-qlatin1string/2
* https://woboq.com/blog/qstringliteral.html
* https://www.qt.io/blog/2014/06/13/qt-weekly-13-qstringliteral

tl;dr: QLatin1Literal uses less memory than QStringLiteral; however,
since most Qt APIs require a QString argument, there's extra runtime
cost of converting QLatin1Strings to QStrings. QStringLiteral uses a
little more memory but constructs its QStrings at compile time.

ok, the `prefer-qstringliteral` branch is getting out of control: the
secondary goal of fixing a .clang-tidy issue is causing more diffs
than the primary goal. So, I'm breaking it into two separate PRs.

qt/AddData.cc

@@ -39,7 +39,7 @@ int AddData::set(QString const& key)
     }
     else if (Utils::isHexHashcode(key))
     {
-        magnet = QString::fromUtf8("magnet:?xt=urn:btih:") + key;
+        magnet = QStringLiteral("magnet:?xt=urn:btih:") + key;
         type = MAGNET;
     }
     else