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.
Initiate the "about middleware" application for Linux
Check uname and show the output
Split version number up into version number and build id
Use "" rather than <> for includes in the current directory
Show the build date, too
Make URL look good.
Reorder

The bitness is more interesting than the rest of the uname data
Don't forget to end gtk_list_store_set() calls properly
Show distribution-related information

call lsb_release -<something> -s and show the result
Make sure the UI definition is shipped
Unbreak distcheck
Look for and display location of PKCS#11 modules
Use the libdir which was specified to configure rather than guessing
Remove no longer needed file
Ignore generated files
Whoops
Update packaging
Look for, and show, the viewer
Don't show the window until it's fully built.

This might have a slight performance impact, but is mostly useful for
debugging, really.