Make insertLineAbove behaviour consistent when at top of document.

Currently, pressing Ctrl+Shift+Enter to insert a line above the current
line does not work consistently when the current line is the first in
the document. For example, if the cursor (|) is not on the first line:

1. #include <QDebug>
2.|#include <QTest>

becomes

1. #include <QDebug>
2.|
3. #include <QTest>

after Ctrl+Shift+Enter. When the cursor is on the first line, however:

1.|#include <QDebug>
2. #include <QTest>

becomes

1. #include <QDebug>
2.|
3. #include <QTest>

after Ctrl+Shift+Enter. This patch corrects the above result:

1. |
2. #include <QDebug>
3. #include <QTest>

This is also in line with Eclipse's behaviour, for example.

Change-Id: I542050b6090ed0cfdf613bf67bbd2651eb99ec9d
Reviewed-by: David Schulz <david.schulz@digia.com>