Document all visual missing commands with nice errors

At least it's nicer for the user than failing or worse.
.
Improve encapsulation of XVimTextObjectEvaluator

only have a "motion" selector to return the computed motion, instead of
everybody doing that.

Also improve the status line when the text evaluator is active in visual
mode: keep printing -- VISUAL* -- and do not forget to reset the argument
string if the text evaluator failed.

Do not fail the visual mode if the motion fails, or a g command fails,
which is what vim does (it also does a bell in that case, but oh well).
Various memory leaks fixed
First stab at block edit: v_b_[AcCI]

Improve the XVimInsertEvaluator to bracket insert operations with undo
groups, it makes undo more predictibly behave like vim does.

remains: v_b_[<>r]