app-office/gnucash: Bump to 2.6.19

app-office/gnucash: Bump to 2.6.19

Added myself as a maintainer.

Added dependency to prevent >=dev-scheme/guile-2.2 from being
considered as a satisfying depend. The GnuCash 2.6 series is only compatible
with guile-1.8 and guile-2.0. guile-2.2 support will come with the
GnuCash 2.7/3.0 series.

Added dependency on gnome-extra/yelp so that GnuCash can actually
display the help documentation.

Bug: https://bugs.gentoo.org/627010
Bug: https://bugs.gentoo.org/639786
Closes: https://bugs.gentoo.org/620314
Package-Manager: Portage-2.3.13, Repoman-2.3.3