LayerSurface: handle focus when switching outputs

Currently if a layer surface is focused and the user focuses a different
output the layer surface remains focused. However, updating focus on
layer surface unmap only considers seats that have the layer surface's
output focused.

To fix this there are 3 approaches I see:

1. Unfocus all layer surfaces on the old output when switching output
focus, focus any layer surfaces on the new output.

2. Disallow switching output focus while a layer surface is focused.

3. Stop caring about output focus when determining which layer surface
should gain/lose focus.

I've taken the 3rd option here as it is significantly simpler to
implement and maintain but still feels reasonably intuitive.

river/LayerSurface.zig

@@ -166,9 +166,6 @@ fn handleKeyboardInteractiveExclusive(output: *Output) void {
     while (it) |node| : (it = node.next) {
         const seat = &node.data;
 
-        // Only grab focus of seats which have the output focused
-        if (seat.focused_output != output) continue;
-
         if (topmost_surface) |to_focus| {
             // If we found a surface that requires focus, grab the focus of all
             // seats.