[FIX] web: improve search in ExportDialog

This commits aims to improve the search feature in the export dialog.
Now, the search isn't resetted when a field is added to the export list
anymore. It is also possible to search in the list of fields using the
technical name of the field.

A test has been added for the search feature in debug mode, and another
one has been adapted to verify that the search input still contains the
string previously entered.

task-3266632

closes odoo/odoo#118398

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>