vte: support temporary charsets and g0-g3

vte: support temporary charsets and g0-g3

Temporary charsets are only used for the next graphics character that is
displayed. After that, the VTE returns to the previous charset. On
soft-reset we also reset these temporary slots.

This also adds the g0-g3 backend slots that can be set by the client and
allow mapping of arbitrary other charsets into gl/gr/glr/grt.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>