Advise kill-buffer to provide more options

Add the option to diff a modified buffer against the file it is visiting
as part of the kill-buffer prompt.

TODO: This needs to be revised to allow scrolling the diffs.
Minor settings tweaks.

Faster completion from company mode; change the color of the mode line
for inactive windows.
Add package: deft
Update org-append-to-subtree

Expand the subtree while jumping to the end.
Add package: helm
Add package: wrap-region
Stop usin bjump for other-window

TODO: Need to assign a different key, or come up with a different rule
for when the bjump targets are shown. It turns out most of the time with
three windows the normal other-window behavior is what I want.
Ibuffer tweaks

Fix C-x C-f behavior from ibuffer -- seems this was dropped by a
previous commit.

Add a key binding to quickly filter the buffer list to buffers visiting files.
Minor cleanup
Add minor mode to protect global keys

Define a minor mode for global key bindings that should not get shadowed
by other major modes.

For example, the M-x keys I use for window split commands wer being
shadowed in magit and diff mode. Moving them to a minor mode keymap
prevents this.
Add transpose-windows command

Also remove unused function.