1. Remove useless props category on component `ChannelMemberListCategory`
2. Make fields of model `DiscussSidebarCategory` more declarative,
i.e. by defining values from compute of field rather than imperative
code somewhere in the code of discuss.
3. Introduce a new model `PopoverManager`, which eases use of `PopoverView`s
as they are automatically added in the DOM in a similar way than `Dialog`s
with `DialogManager` or `ChatWindow`s with `ChatWindowManager`.
Doing so also make these popover view no longer deep child in component tree:
this fixes an issue in which popover views could be cut due to DOM tree.
Task-2871688
closesodoo/odoo#92896
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>