[WIP] giscanner: include linemarkers in "gir/[gio|glib|gobject]-2.0.c"

Makes it obvious where a GTK-Doc comment block was picked up.

Note that these are not C pre-processor style linemarkers on
purpose to prevent g-ir-scanner's SourceScanner() from tripping
up over the non-absolute paths in the generated
"gir/[gio|glib|gobject]-2.0.c" files.
[WIP] docs: improve layout
!!! for testing only !!!