fix(winfloat): last_status when changing split to floatwin

Problem: converting a split to a floatwin may not remove the last statusline
when needed. (e.g: 'ls' is 1)

Solution: call last_status/win_comp_pos in win_new_float, after win_remove.

Also fix float_pos formatting for screen snapshots so it doesn't give a nil
error for external windows.

Not an issue from this PR.

test/functional/ui/float_spec.lua

@@ -7758,12 +7758,11 @@ describe('float window', function()
           screen:expect {
             grid = [[
           ## grid 1
-            [2:----------------------------------------]|*5
-            {5:[No Name] [+]                           }|
+            [2:----------------------------------------]|*6
             [3:----------------------------------------]|
           ## grid 2
             x                                       |
-            {0:~                                       }|*4
+            {0:~                                       }|*5
           ## grid 3
                                                     |
           ## grid 4