Support vim's gJ and v_gJ

This just joins lines without playing tricks with spaces
Positions after undo or gv are still a mess though.
Add +popEvaluator

When an evaluator returns +popEvaluator it means that it shall be popped
and the parent evaluator should go on.
DEL/FORWARD_DELETE should actually be BS/DEL in vim's terminology

Fixes: #523
Document all visual missing commands with nice errors

At least it's nicer for the user than failing or worse.