QmlDesigner.PropertyEditor: Give more space to color LineEdit

Task-number: QTCREATORBUG-11992

Change-Id: Icb884847633bd421e3dc7bc034d598ef84d33422
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>