When using the LiteHTML Viewer plugin to view an email's HTML part (either in message view or by opening it in a new window), keyboard navigation (space bar,
Enter, PgUp, PgDn, up/down arrows) doesn't work, whereas it does when viewing other part types.

Steps to Reproduce:

1) Install Claws Mail 3.17.4 from source after using package manager to install 'libetpan' and 'libgumbo' libraries from Ubuntu 18.04 repos. (Basic ./configure command, other than using an alternative directory for the config.)

2) View the HTML part of an email that has one, either in message view or by opening in a new window.

3) Try to use the keyboard to scroll up and down in the email as you would when viewing a different email part (i.e. space bar, Enter, etc.).

Actual Results: No response to keyboard inputs. (But mouse can be used with the scroll bar as a workaround.)

Expected Results: The message should move up and down according to keyboard inputs, consistent with how it does when viewing other parts.

I know that this plugin has some issues that are related to the limitations of 'litehtml' itself, but this seems like it might be something that could be fixed in the plugin code itself.