Scrolling in help screen

With a lot of features added into htop over time, htop's help screen (the screen you see when you press the h or F1 key) is already unable to present all its contents into the 80×24 screen size that most terminal emulators still use by default.

The current (as of 2025-01-01) htop help screen requires at least 32 lines to display:

This feature request is useful for new people who can contribute to htop.

As the hotkeys in htop will likely be more, not less over time, it might be a good time to make htop help screen scrollable now. And perhaps ideally, adapted to different terminal screen sizes (some screens in embedded systems can be very small).

Goals

• Allow htop help screen to scroll, with the minimum support of Up, Down, PageUp, PageDown, and Space keys.
• Space key behavior changed from exiting the help screen to scrolling the help screen down by one page. If the scrolling window is at the bottom of the help page, the Space key will exit the help screen. (The Enter key can have identical behavior.)
• q and Escape keys will exit the help screen.
• I can't tell we need horizontal scrolling at this time, but help screen needing horizontal scrolling would be bad in usability, so I don't say Left or Right key as part of the goals.

Secondary goals

• Add a Function Bar at the bottom of the terminal screen, explaining the keys that can be used to scroll. The items on the Function Bar should have a minimum of these:

PgUp  PgDnScrollEscDone
    --    ------   ------

• The Function Bar should ideally allow mouse clicks.

Tertiary goals (not really necessary but it's good to have it nevertheless)

• Make the list of hotkeys shown in help screen adaptable to smaller screen sizes. Currently it's two columns, but on a smaller screen, make it able to display as one column instead. This may require reorganization of the hotkey items, thus it's not an easy task to be done.