Fixed handling of text wrapping and a text layout corner case

Due to using QTextCursor::columnNumber() instead of
QTextCursor::positionInBlock(), a lot of code would not work correctly
when used with wrapped lines.

In addition, there was an issue with columnNumber() returning 0 right
after inserting a character before the last character of a line.

Reviewed-by: mae

src/plugins/qmljseditor/qmlexpressionundercursor.cpp

@@ -102,7 +102,7 @@ public:
         _cursor = cursor;
 
         QTextBlock block = _cursor.block();
-        const QString blockText = block.text().left(cursor.columnNumber());
+        const QString blockText = block.text().left(cursor.positionInBlock());
 
         scanner.setScanComments(false);
         const QList<Token> tokens = scanner(blockText, startState(block));