2D matrix for x2many fields

This module allows to show an x2many field with 3-tuples (\$x_value,
\$y_value, \$value) in a table

|            | \$x_value1   | \$x_value2   |
|------------|--------------|--------------|
| \$y_value1 | \$value(1/1) | \$value(2/1) |
| \$y_value2 | \$value(1/2) | \$value(2/2) |

where value(n/n) is editable.

An example use case would be: Select some projects and some employees so
that a manager can easily fill in the planned_hours for one task per
employee. The result could look like this:

![Screenshot](https://raw.githubusercontent.com/OCA/web/12.0/web_widget_x2many_2d_matrix/static/description/screenshot.png)

The beauty of this is that you have an arbitrary amount of columns with
this widget, trying to get this in standard x2many lists involves some
quite ugly hacks.