#1211 Add C::B manual to Linux help menu like Windows

[Andrew Cottrell]

On Linux C::B does not include the C::B manual. The PDF C::B manual and CHM should IMHO both be added.

Linux could include the CHM files, but the end user would need to install a CHM viewer like one mentioned in the following article:
https://blog.desdelinux.net/en/como-abrir-archivos-chm-en-linux/
I tested xchm on XUbuntu 20.04 and it works with the C::B manual.

[Andrew Cottrell]

This patch adds the docs to the Linux build process and also fixes ticket 1213 for the src/plugins/contrib/help_plugin/help_common.cpp as it was needed to test the changes, so I included it in this patch.

This patch does not check if the help files can be opened on Linux or OSX.

[Miguel Gimenez]

Fixed in [r12725].

Can [#1213] be closed?

[Andrew Cottrell]

Tickets: #1213 cannot be closed as there are other files that need fixing. This fixes just one of them as it was need to get the help menu working.