Add the `scroll-command' property.

Add the `scroll-command' property.

* window.c: Add Qscroll_command.
Remove Vscroll_preserve_screen_position_commands.
(window_scroll_pixel_based, window_scroll_line_based): Check the
`scroll-command' property on the last command instead of searching
the last command in Vscroll_preserve_screen_position_commands.
(syms_of_window): Initialize and staticpro `Qscroll_command'.
Put Qscroll_command property on Qscroll_up and Qscroll_down.
(scroll-preserve-screen-position): Doc fix.
(Vscroll_preserve_screen_position_commands): Remove variable.

* simple.el (scroll-up-command, scroll-down-command)
(scroll-up-line, scroll-down-line): Put `scroll-command'
property on the these symbols.  Remove them from
`scroll-preserve-screen-position-commands'.

* mwheel.el (mwheel-scroll): Put `scroll-command' and
`isearch-scroll' properties on the `mwheel-scroll' symbol.
Remove it from `scroll-preserve-screen-position-commands'.

* isearch.el (isearch-allow-scroll): Doc fix.