Fix (partially) actions position when Talk is embedded in the Files app

Using the default container causes the action menus in the Talk tab to
be repositioned at wrong places when the menus are shown and the file
list is scrolled. To address this (although it does not fully fix the
issue, there are still some strange behaviours) the main container for
Talk components used when Talk is embedded in the Files app is Talk tab.
Besides that, both the container and the boundaries element of the
actions are set to the Talk tab.

Despite setting the main container this change does not affect other
components (like the room selector) or slightly improves their behaviour
(like the author avatar menu, which no longer appears outside the tab
when scrolling).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>