Simplify adding attachment

Adding an attachment could be complicated for the user because we open an editable list in a dialog.
Indeed we could simplify the common actions of adding a file and opening existing attachment. The attachment could be a dropdown menu with the attachments, clicking on it to open and an entry to add or manage.

assigned to @ced

added sao tryton trytond type::feature + 1 deleted label

New review50551002 at https://codereview.tryton.org/50551002/#ps1

review50551002 updated at https://codereview.tryton.org/50551002/#ps20001

review50551002 updated at https://codereview.tryton.org/50551002/#ps40001

added 1 deleted label and removed 1 deleted label

review50551002 updated at https://codereview.tryton.org/50551002/#ps60001

review50551002 updated at https://codereview.tryton.org/50551002/#ps80001

review50551002 updated at https://codereview.tryton.org/50551002/#ps100001

review50551002 updated at https://codereview.tryton.org/50551002/#ps120001

review50551002 updated at https://codereview.tryton.org/50551002/#ps140001

review50551002 updated at https://codereview.tryton.org/50551002/#ps160001

New changeset f742d164e872 by Cédric Krier in branch 'default':
Use dropdown for attachments
https://hg.tryton.org/sao/rev/f742d164e872

added 1 deleted label and removed 1 deleted label

closed

New changeset 4d111f5205a2 by Cédric Krier in branch 'default':
Use dropdown for attachments
https://hg.tryton.org/tryton/rev/4d111f5205a2

New changeset 8a18a7977f9a by Cédric Krier in branch 'default':
Use dropdown for attachments
https://hg.tryton.org/trytond/rev/8a18a7977f9a

New changeset 1339844f41a2 by Cédric Krier in branch 'default':
Use dropdown for attachments
https://hg.tryton.org/tryton-env/rev/1339844f41a2

mentioned in issue #7662 (closed)

mentioned in issue #7735 (closed)