Don't create a QString for every char in decodeLine

PlainTextDecoder::decodeLine() was creating a new QString via
QString::fromUcs4() for every character.  Instead, accumulate the
characters in a QVector<uint>, and call QString::fromUcs4() once at the
end of the function.

src/decoders/PlainTextDecoder.cpp

@@ -69,12 +69,6 @@ void PlainTextDecoder::decodeLine(const Character *const characters, int count,
 
     // TODO should we ignore or respect the LINE_WRAPPED line property?
 
-    // note:  we build up a QString and send it to the text stream rather writing into the text
-    // stream a character at a time because it is more efficient.
-    //(since QTextStream always deals with QStrings internally anyway)
-    QString plainText;
-    plainText.reserve(count);
-
     // If we should remove leading whitespace find the first non-space character
     int start = 0;
     if (!_includeLeadingWhitespace) {
@@ -109,14 +103,19 @@ void PlainTextDecoder::decodeLine(const Character *const characters, int count,
         }
     }
 
+    // note:  we build up a QVector<uint> and send it to the text stream transformed into a QString
+    // rather than writing into the text stream a character at a time because it is more efficient.
+    //(since QTextStream always deals with QStrings internally anyway)
+    QVector<uint> characterBuffer;
+    characterBuffer.reserve(count);
+
     for (int i = start; i < outputCount;) {
         if ((characters[i].rendition & RE_EXTENDED_CHAR) != 0) {
             ushort extendedCharLength = 0;
             const uint *chars = ExtendedCharTable::instance.lookupExtendedChar(characters[i].character, extendedCharLength);
             if (chars != nullptr) {
-                const QString s = QString::fromUcs4(chars, extendedCharLength);
-                plainText.append(s);
-                i += qMax(1, Character::stringWidth(s));
+                characterBuffer.insert(characterBuffer.end(), extendedCharLength, *chars);
+                i += qMax(1, Character::stringWidth(chars, extendedCharLength));
             } else {
                 ++i;
             }
@@ -129,12 +128,12 @@ void PlainTextDecoder::decodeLine(const Character *const characters, int count,
             // lost in some situation. One typical example is copying the result
             // of `dialog --infobox "qwe" 10 10` .
             if (characters[i].isRealCharacter || i <= realCharacterGuard) {
-                plainText.append(QString::fromUcs4(&characters[i].character, 1));
+                characterBuffer.append(characters[i].character);
                 i += qMax(1, characters[i].width());
             } else {
                 ++i; // should we 'break' directly here?
             }
         }
     }
-    *_output << plainText;
+    *_output << QString::fromUcs4(characterBuffer.data(), characterBuffer.size());
 }