Fix EZP-23211: Impossible to align multiple cells

JIRA: https://jira.ez.no/browse/EZP-23211

# Description

At the moment, it's impossible to align several table cells in Online Editor. This is happening because the selection is not correctly handled when the user selected several table cells. To workaround that, the table plugin (and our fork eztable) sets the `mceSelected` class on the selected cells/rows; so we just have to check that to detect the elements to align.

# Tests

manual tests