Added spaces as an invisible
Add layout_t::set_character_mapping

This allow configuring which characters should be used to render tabs, spaces, and newlines (when “show invisibles” is enabled).
Add “invisiblesMap” option to the .tm_properties file

This setting allows changing which invisibles are shown and what glyph
is used for them.

Invisibles are '\t', '\n', and ' ', to turn one off, add ~[\t \n] to
the settings string. To set the glyph used for the invisible, add [\t
\n][glyph_to_use].