Improve binary widget buttons
Similar to #4540 (closed), the set of buttons could be reworked to minimize errors.
So I propose to merge the clear/select button and move it at the end so it stays at the same place when clear is clicked so select can be clicked just after without moving.
I propose also to move the open button inside the filename entry as primary icon, like for the Many2One.
And I propose to hide the download button when there is no value.