QmlJsEditor: unify behavior of autocompletion to C++ editor.

Changed behavior of autocompletion for uniformity with C++ completion.
E.g: changing 'x' to 'c' in "Rextangle" used to trigger completion,
resulting in: "Rectangletangle".

Task-number: QTCREATORBUG-15379
Change-Id: I0597a8d76843b8f1c20c39877267aa70a6b0e1a2
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/plugins/qmljseditor/qmljsreuse.cpp

@@ -54,6 +54,15 @@ bool isIdentifierChar(const QChar &c, bool atStart, bool acceptDollar)
     }
 }
 
+bool isValidFirstIdentifierChar(const QChar &ch) {
+    return ch.isLetter() || ch == QLatin1Char('_') || ch.isHighSurrogate()
+            || ch.isLowSurrogate();
+}
+
+bool isValidIdentifierChar(const QChar &ch) {
+    return isValidFirstIdentifierChar(ch) || ch.isNumber();
+}
+
 bool isDelimiterChar(const QChar &c)
 {
     switch (c.unicode()) {