Revert "Switch gtk2 to gtk3"

This reverts commit 4ad5e7e6c628c862ea11e8307fe51043780df046.

RedHat 6 (which we still do support) does not have Gtk3, so we can't
require it yet.
Add start of C++ eid-viewer
Updates
Add gettext to the picture
Add support for threading, and an initial C_WaitForSlotEvent code
Make part of the license text translatable
Fix text so it is in line with current implementation
Cosmetic change: make sure the label is aligned properly
Implement translation logic
Don't forget to add the gettextize output
Create an accel group. Not quite working yet, though.