[FIX] web_editor: fixed issue of chrome triple click

Partial backport of 11.0 commit (https://github.com/odoo/odoo/commit/6ad2ae30354a77c0f898cb9cb5a859713061d71a)
to solve issue 774152. Note: it has to be forward-ported as
implementation has been improved to handle mass mailing cases.

When an user triple-clicked on a paragraph and then typed text, the
paragraph was removed instead of its content (so the new text was
added at the beginning of the next paragraph).

See: http://stackoverflow.com/questions/38467334/why-does-google-chrome-always-add-space-after-selected-text