Prior to this commit, read-only handles were hidden, which created a
layout issue (eg. the header was no longer highlighted when we clicked
on it).
This commit adapts read-only handles to make them visible by using a
border around the selected element.
It also adapts the common borders of all handles to place it
inside the element to avoid being partially hidden when the element is
on the edge of the container (eg. the header or the footer).
task-3537616
closesodoo/odoo#137569
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>