Fixes#1438.
Give the user the ability to override the functionality of the cancel button by passing in their own function. If no override is provided, the existing functionality of reseting selected row ids is preserved.
Example implementation:
```
<ToolbarBatchActions cancel={()=>{console.log("User clicked cancel!")}}>
...
</ToolbarBatchActions>
```
* fix(combo-box): clicking chevron should not open the combo box if disabled
Fixes#776
* fix(toolbar-batch-actions): cancel text should be slottable
Fixes#782