Rewrite con_swap to work only with queue operations

Benefits are that we don't open a fake container and don't call many
complicated functions that can lead to redraws (x_push_changes calls) as
discussed in #2954.

Fixes #2810:
Windows exchange floating mode & window rects.
Swap will still not work with CT_FLOATING_CONs but this doesn't make
much sense.

Fixes #3280:
The behaviour is not very user friendly but swap behaves exactly as it
should. The rest is a tree_flatten issue. Attached pictures in #2954.

docs/userguide

@@ -2092,8 +2092,7 @@ using one of the following methods:
 +mark+:: A container with the specified mark, see <<vim_like_marks>>.
 
 Note that swapping does not work with all containers. Most notably, swapping
-floating containers or containers that have a parent-child relationship to one
-another does not work.
+containers that have a parent-child relationship to one another does not work.
 
 *Syntax*:
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