* mh-tool-bar.el (image-load-path): Define to shush compiler.

* mh-tool-bar.el (image-load-path): Define to shush compiler.
(mh-buffer-exists-p): Move inside mh-do-in-gnu-emacs since it isn't
used outside of it.
(mh-tool-bar-folder-buttons-init, mh-tool-bar-letter-buttons-init):
Update load-path/image-load-path before setting buttons. This code
used to be in mh-folder-mode/mh-letter-mode but this was the wrong
place since mh-tool-bar-*-buttons-init can also be called when
customizing the buttons.
(mh-tool-bar-update): New function which updates tool-bar-map in all
of the MH-E buffers after customizing the buttons (closes SF
#1452718).
(mh-tool-bar-folder-buttons-set, mh-tool-bar-letter-buttons-set): Call
it (closes SF #1452718).

* mh-folder.el (mh-folder-buttons-init-flag): Delete. Use
mh-folder-tool-bar-map instead.
(image-load-path): Delete. No longer used.
(mh-folder-mode): Moved setting of image-load-path into
mh-tool-bar-folder-buttons-init.

* mh-letter.el (mh-letter-buttons-init-flag): Delete. Use
mh-letter-tool-bar-map instead.
(image-load-path): Delete. No longer used.
(mh-letter-mode): Moved setting of image-load-path into
mh-tool-bar-letter-buttons-init.

* mh-seq.el (mh-narrow-to-seq, mh-widen): Use with-current-buffer
instead of set-buffer.