Allow to undo and redo selection changes

From the issue:

> It often happens to me that I carefully craft a selection with multiple
> cursors, ready to make changes elegantly, only to completely mess it
> up by pressing a wrong key (by merging the cursors for example). Being
> able to undo the last selection change (even if only until the previous
> buffer change) would make this much less painful.

Fix this by recording selection changes and allowing simple linear
undo/redo of selection changes.

The preliminary key bindings are <c-h> and <c-k>.
Here are some other vacant normal mode keys I considered

	X Y
	<backspace> <minus>
	# ^ =
	<plus> '

unfortunately none of them is super convenient to type.  Maybe we
can kick out some other normal mode command?

---

This feature has some overlap with the jump list (<c-o>/<c-i>) and
with undo (u) but each of the three features have their moment.

Currently there's no special integration with either peer feature;
the three histories are completely independent.  In future we might
want to synchronize them so we can implement Sublime Text's "Soft
undo" feature.

Note that it is possible to restore selections that predate a buffer
modification. Depending on the buffer modification, the selections
might look different of course. (When trying to apply an old buffer's
selection to the new buffer, Kakoune computes a diff of the buffers
and updates the selection accordingly. This works quite well for
many practical examples.)

This makes us record the full history of all selections for each
client. This seems wasteful, we could set a limit. I don't expect
excessive memory usage in practice (we also keep the full history of
buffer changes) but I could be wrong.

Closes #898

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/pages/keys.asciidoc b/doc/pages/keys.asciidoc
index 73f0e432..bce3b070 100644
--- a/doc/pages/keys.asciidoc
+++ b/doc/pages/keys.asciidoc
@@ -314,6 +314,12 @@ Yanking (copying) and pasting use the *"* register by default (See <<registers#,
 *<a-U>*::
     move forward in history
 
+*<c-h>*::
+    undo last selection change
+
+*<c-k>*::
+    redo last selection change
+
 *&*::
     align selections, align the cursor of each selection by inserting spaces
     before the first character of each selection