Incorrect cursor position after delete table cells

[garry.yao]

Reproducing Procedures

1. Open 'replacebyclass' sample page insert an default table.
2. Click inside one of the table cell and open context menu on it.
3. Click 'Cell->Delete Cells' to remove it.

• Actual Result: The cursor is not blinking at the same position agreed on all browsers, especially for some one the cursor is lost.

[garry.yao]

Similar with #4851.

[garry.yao]

1. Load the following content and selection into editor in Firefox:

   	<table>
   		<tr><td>[cell1]</td><td>cell2</td></tr>
   		<tr><td>[cell3]</td></tr>
   	</table>
   

2. Delete the selected cells using context menu.

• Expected Result: Cursor is blinking inside 'cell2'.
• Actual Result: Cursor is not blinking any more.

[brooks]

update for 2 bugs:

1. fix the algorithm for find the first focusable cell.

2. if the last cell was deleted, delete the whole table.

[garry.yao]

1. fix the algorithm for find the first focusable cell.

With more robust logic right now, the patch increases the complexity of the cells deletion to O(n²) because of the 'indexOf' search, It's our common practice to use ​this way to filtering unwanted nodes, which could reduce to linear O(2n).

[brooks]

nice suggest!

[garry.yao]

Congs! The codes looks much neater.

[brooks]

fixed with 5352