bug#29322: Isearch toolbar

[Charles A. Roelli]

Since the default toolbar map is not useful during Isearch, we could
show special toolbar buttons during Isearch, to allow navigation
forward and backward between matches, to start the search from the
top/bottom of the buffer, to restart the search backward from
window-start or forward from window-end (compare the "p" and "n"
commands in view-mode), and maybe for toggling case/character folding
and lazy highlighting.