Remove scroll mode which is now redundant, copy mode should be used instead.

The = key binding now does nothing.

Permit options such as status-bg to be configured using the entire 256 colour
palette by setting "colour0" to "colour255".

vi(1)-style half page scroll in copy and scroll modes. Move the vi full page
scroll key to C-b instead of C-u and use C-u/C-d for half page scrolling with
vi keys. In emacs mode, half page scrolling is bound to M-Up and M-Down.

Suggested by merdely (about a year ago :-)).

Scroll by two less than the number of lines in the screen, like emacs, rather
than by the entire screen, to make it easier to pull things out from under the
line indicator. Suggested by claudio.

If colours are not supported by the terminal, try to emulate a coloured
background by setting or clearing the reverse attribute.

This makes a few applications which don't use the reverse attribute themselves
a little happier, and allows the status, message and mode options to have
default attributes and fg/bg options that work as expected when set as reverse.

Next step towards customisable mode keys: build each default table of keys
into a named tree on start and use that for lookups. Also add command to string
translation tables and modify list-keys to show the mode key bindings (new
-t argument).

Remove an unused entry in the mode keys command enum and rename
MODEKEYCOPY_QUIT to _CANCEL to match the others.

Change mode key bindings from big switches into a set of tables. Rather than
lumping them all together, split editing keys from those used in choice/more
mode and those for copy/scroll mode.

Tidier and clearer, and the first step towards customisable mode keys.

Get rid of empty mode_key_free function.