Don't deprecate gtk_widget_show_all()

Don't deprecate gtk_widget_show_all()

After discussions on IRC, the conclusion was reached that deprecations
only make sense if an action can be taken to not use the deprecated code
that makes the code more current and simplifies a later port to a newer
GTK version.

In this particular case, the suitable action would be adding
gtk_widget_show() calls whenever a widget is created, so that a call to
show_all() is not necessary.
However, in GTK4 these calls would not be necessary and end up just
bloating the codebase unnecessarily.

So it was decided the better solution would be to not deprecate the API
and instead leave this work to be done during potential GTK4 ports of
applications.

This reverts commit 4d71d2303dafe1c02eb79040e32fe2172d1f295f.

Fixes !1282