-fixed a bug when one selects text from right to left and then shifts it (see comment "in case selection_start was after selection_end" in my code)
This seems to already have been fixed by @b4n in dc0af51b96d4b98ada550d464b0cd17b24268419. I cannot reproduce any issue on selecting text from the right and shifting it. Works fine.
-When no text is selected, now the current word is exchanged with the next/previous, which I find quite useful.
IMHO this does not fit into this plugin. If someone needs it then a new PR to a different plugin where it fits in or a new plugin is welcome.
I vote for closing this un-merged. What do the others think?