Always show message action menu

[gtandersen]

Is your feature request related to a problem? Please describe.
When performing actions on messages in the list, the cursor has to travel a varying distance/direction to reach the "floating" message action menu that appears to the right in the message list column.

Additionally, it can be confusing for new users that the menu isn't shown until one or more messages are selected.

Describe the solution you'd like
A persistent message action menu at the top of the message list in combination with the search area. The floating menu can be retained for small (mobile) screens in the PWA.

See also #1394

[gtandersen]

Fixed in #1495.