修复Extjs5.1.4表格设置enableTextSelection: true之后，文本仍然不能选择的BUG

如果您发现其他版本也有此BUG，可参照此方式进行修复，源代码中多了一句拦截mousedown事件的代码mousedownEvent.preventDefault()造成的。

Ext.define(‘Ext.override.grid.NavigationModel‘,{    override:‘Ext.grid.NavigationModel‘,    onCellMouseDown: function(view, cell, cellIndex, record, row, recordIndex, mousedownEvent) {        var parentEvent = mousedownEvent.parentEvent,            targetComponent = Ext.Component.fromElement(mousedownEvent.target, cell),            column = mousedownEvent.position && mousedownEvent.position.column,            cellEditing = view.grid.findPlugin(‘cellediting‘);

if (targetComponent && targetComponent.isFocusable && targetComponent.isFocusable()) {            if (!cellEditing || !cellEditing.editing) {                targetComponent.focus();            }            if (column && column.stopSelection !== false) {                return;            }        }        if (!parentEvent || parentEvent.type !== ‘touchstart‘) {            this.setPosition(mousedownEvent.position, null, mousedownEvent);        }    }});