Avoid some use of (deprecated) Gtk::Alignment in code.

Use Gtk::Bin for placeholder parent widgets in the code instead,
though they are still GtkAlignment in the Glade .ui files.
Replace more Gtk::Alignment uses with Gtk::Widget margin and alignment.
Revert "Adapt to the libgda-6.0/libgdamm-6.0 API."

This reverts commit 9382d0e046a64a488f93d0473b30ec90a0fc2711.
Replace most uses of Gtk::Alignment in the code.