Merge bitcoin-core/gui#313: qt: Optimize string concatenation by default

a02c970eb0 qt, refactor: Revert explicit including QStringBuilder (Hennadii Stepanov)
3fd3a0fc87 qt, build: Optimize string concatenation (Hennadii Stepanov)

Pull request description:

  From [Qt docs](https://doc.qt.io/qt-5/qstring.html#more-efficient-string-construction):
  > ... multiple uses of the \[`QString`\] '+' operator usually means multiple memory allocations. When concatenating n substrings, where n > 2, there can be as many as n - 1 calls to the memory allocator.

  With this PR
  > ... the '+' will automatically be performed as the `QStringBuilder` '%' everywhere.

  The change in the `src/Makefile.qt.include` file does not justify submitting this PR into the main repo, IMHO.

ACKs for top commit:
  laanwj:
    Code review ACK a02c970eb0
  Talkless:
    utACK a02c970eb0, built successfully on Debian Sid with Qt 5.15.2, but did not check if any displayed strings are "wrong" after refactoring.
  jarolrod:
    ACK a02c970eb0

Tree-SHA512: cbb476ee96f27c3bd6e125efab74d8bf24bbdb4c30576b3feea45e203405f3bf5b497dd7d3e11361fc825fcbf4b893b152921a9efdeaf73b42d1865d85f0ae84

src/qt/optionsmodel.cpp

@@ -21,6 +21,7 @@
 #include <validation.h> // For DEFAULT_SCRIPTCHECK_THREADS
 
 #include <QDebug>
+#include <QLatin1Char>
 #include <QSettings>
 #include <QStringList>
 
@@ -244,7 +245,7 @@ static ProxySetting GetProxySetting(QSettings &settings, const QString &name)
 
 static void SetProxySetting(QSettings &settings, const QString &name, const ProxySetting &ip_port)
 {
-    settings.setValue(name, ip_port.ip + ":" + ip_port.port);
+    settings.setValue(name, QString{ip_port.ip + QLatin1Char(':') + ip_port.port});
 }
 
 static const QString GetDefaultProxyAddress()