[BITV/A11Y] ActionMenu accessibility

[PVince81]

• Menu opening button does not need aria-haspopup because it is not a role="menu", but a hasmenu, and is deprecated. Can be removed because we have aria-expanded
• Menu should be inserted in the DOM directly behind the button itself → this would also fix the bug where in the files list on scrolling, the menu scrolls away.
• aria-label property is not needed as there is a text label already, can be removed.
• Focus should initially stay on the toggle button, not move into first entry of the list
• Focus should not loop through the menu but after last entry, close the menu and move to next
• Same with Shift-Tab, should move back to toggle button after first entry
• Focus needs to get border

[JuliaKirschenheuter]

Menu-related requirements are described here nextcloud/server#37099
Menu-related focus trap is described in other known issues, like nextcloud/server#41012
Focus border was discussed in other issues, not needed to be applied

Moving issue to "descoped" for now

[JuliaKirschenheuter]

closing as done