Revert "vte3-gi: define constants under Terminal"

This reverts commit 6ff9c5186d04ed9af0c774549c71a5e6138ba194.

It is for avoiding deep class tree.
ref: http://ruby-gnome2.sourceforge.jp/hiki.cgi?Naming+and+Conversion+Rules#Class%2FModule+names
Revert "vte3-gi test: modify test for `define constants under Terminal`"

This reverts commit 68ebd26985905bdef543245cd9a414b2ac51cb4d.

Avoiding deep class tree.