Add support for vertical text writing.

Add new 4 GIMP TEXT DIRECTION. TTB-LTR, TTB-LTR-UPRIGHT, TTB-RTL, TTB-RTL-UPRIGHT. See also #641 (closed).

Squashed commit of the following:

commit ee1ff7d502658cfa1248a13a3f0348495db07eda Author: ONO Yoshio ohtsuka.yoshio@gmail.com Date: Sun Jul 29 00:31:47 2018 +0900

Fixed that gimp-text-dir-ttb-* icons are lacked in Symbolic.

commit d87d012d697628da28fe90199cc04b95b72ba8ef Author: ONO Yoshio ohtsuka.yoshio@gmail.com Date: Sat Jul 28 16:23:10 2018 +0900

Fix a typo.

commit cf0238bf7df56c384cdf3b7ec69557d14740f853 Author: ONO Yoshio ohtsuka.yoshio@gmail.com Date: Sat Jul 28 15:50:57 2018 +0900

Fixed seg fault error.

commit b07f60d0 Author: ONO Yoshio ohtsuka.yoshio@gmail.com Date: Fri Jul 27 17:15:34 2018 +0900

Add support for vertical text writing.

https://gitlab.gnome.org/GNOME/gimp/issues/641