Add the keycodes and routines to allow the user to forego setting the
mark explicitly (with M-A / ^6) and instead quickly select a few words
or lines by holding down Shift together with the movement keys.

(Some combinations with Shift are swallowed by some terminal emulators.
To work around some of those, the combinations Shift+Alt+Left/Right work
as Shift+Home/End and Shift+Alt+Up/Down work as Shift+PageUp/PageDown.)

And move a statement to a better place.

This also prevents the message from spilling into the help lines
when the terminal is less than four columns wide.  (Not that one
can read anything then, but it looks better.)

This fixes https://savannah.gnu.org/bugs/?48787.

This fixes https://savannah.gnu.org/bugs/?48520.

This fixes https://savannah.gnu.org/bugs/?48751 reported by Andrew Ho.

This also fixes compilation when configured with --enable-tiny.

If this breaks your build, please send report or instructions or patch.

This fixes https://savannah.gnu.org/bugs/?48711.

(It may have been necessary in the past; there is no need for it now.)

This fixes https://savannah.gnu.org/bugs/?48633.

For horizontal scrolling, it is not the /desired/ column position that
is relevant for determining whether a line needs to be redrawn after a
cursor movement, but the /actual/ column positions before and after.

This fixes https://savannah.gnu.org/bugs/?48627,
and fixes https://savannah.gnu.org/bugs/?48629.

Leave the rest of any escape sequence to be processed normally, which
should be possible because those characters are all in ASCII range.

This fixes https://savannah.gnu.org/bugs/?48318.

These codes are the basic codes that get assigned for the movement keys
in the list of shortcuts -- they don't need any translation.

Again, just for preapring an easier return from the parsing routine.

There is no need to specially flag a function key or editing key --
the keycode is indication enough in itself: outside of byte range.

The keycode is not a sequence; it is a single integer value.

And instead of using two key types, just use a bool to indicate whether
a keystroke involves Meta.

A keycode is either already in byte range (so there is nothing to do)
or it is not and it shouldn't be (so there is nothing to do either).

After initialization, the type of a key is never used nor needed
(other than for meta keys).

Accept the "Esc [ 1 n ~" and "Esc [ 2 n ~" sequences only when they
end with the "~" character, not with any character.

Those lines don't exist in that mode -- the relevant subwindows are
smaller, or have been made smaller just before.

It will be initialized when a new code is started.