[FIX] web_editor: prevent error on font-size change

Before this commit, changing the font size of an element with one
ancestor having the same font size could trigger a traceback. For fixing
that issue some code has to be refactored to handle the setFontSize case
in formatSelection. As a side effect of this refactoring, nested
font-size is now properly handled (see added tests).

task-3086301

closes odoo/odoo#107381

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>