Store a backup copy of input-decode-map for restorability.

Previously, gedit-mode would twiddle your input-decode-map, and then
if you tried to disable gedit-mode, you'd be left with the broken
input-decode-map, but all the gedit keybindings gone, leaving your C-x
and C-c keys unbound and broken. Now I store a backup copy and restore
it during unloading, so you should now be able to call M-x gedit-mode
twice and not notice anything break on you.