Fix `C-x C-f` binding for ibuffer.

Ibuffer overrides the default C-x C-f binding, which means my usual
ido/bookmark stuff doesn't work if I happen to be looking at ibuffer.

Added my/ibuffer-ido-find-file which copies the ibuffer-find-file
implementation but uses ido-completing-read.
Remove call to obsolete toggle-read-only function.
Add undo-tree package.
Tweak ido/bookmark integration.

Disabling flx for the moment as it breaks my use of bookmarks from
ido-find-file.