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/transactiontablemodel.cpp

@@ -25,6 +25,8 @@
 #include <QDateTime>
 #include <QDebug>
 #include <QIcon>
+#include <QLatin1Char>
+#include <QLatin1String>
 #include <QList>
 
 
@@ -413,9 +415,9 @@ QVariant TransactionTableModel::txAddressDecoration(const TransactionRecord *wtx
 QString TransactionTableModel::formatTxToAddress(const TransactionRecord *wtx, bool tooltip) const
 {
     QString watchAddress;
-    if (tooltip) {
+    if (tooltip && wtx->involvesWatchAddress) {
         // Mark transactions involving watch-only addresses by adding " (watch-only)"
-        watchAddress = wtx->involvesWatchAddress ? QString(" (") + tr("watch-only") + QString(")") : "";
+        watchAddress = QLatin1String(" (") + tr("watch-only") + QLatin1Char(')');
     }
 
     switch(wtx->type)