Clean up by introducing a wrapper struct for mouse clicks rather than passing
three u_chars around.
As a side-effect this fixes incorrectly rejecting high cursor positions
(because it was comparing them as signed char), reported by Tom Doherty.
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).
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.
