Fix performance regression when appending one vector to another

Fix performance regression when appending one vector to another

When appending a QVector to an existing vector the code would
unconditionally realloc the vector instead of first checking
whether we can do without. This gives a quadratic behaviour when
repeatedly appending a vector to another.

Change-Id: I2cd81253e6a8aec0bc4402e6fbda262435080966
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>