ui/gtk: require at least GTK 2.18 and VTE 0.26

This gives us the bare amount of features we need.  We can add work arounds
for older versions and lower the requirement but this should be a good
starting point.

Suggested-by: Daniel Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
v1 -> v2
 - tremendous simplification suggested by danpb