We're a normal application, so we can use the normal way to hook up code
into builder files, which is using dlsym() for the function pointers.
There's no need to to extra work exporting static symbols.
gtk_builder_connect_signals() is no longer necessary, because all the
setup that made it necessary to have this extra step is now done
automatically via the closure functions.
This is pretty unused and gets in the way of the next steps.
A potential side effect is that for templates the widget was passed as
the user data argument. If that turns out to be important, we have to
special case that situation.
Use a key controller instead. Note that this currently
breaks the handling of Enter, since we activate key bindings
twice, causing us to switch to the label and back.
Add more features to the list, allow selecting script/language
from the set that is supported by the font, indicate which
features are present in the font for the selected script/language,
and expand the default specimen to cover latin, cyrillic and
greek.
