gtk-demo: Cleanup deprecated calls throughout demo code

Specify constructor arguments as keyword args for various object
creation calls. Update combobox demo to use bind_property instead of manual
signal connections with update callbacks.
gkt-demo: Change main info/source notebook into a GtkStack

Use the new in 3.10 GtkStack and GtkStackSwitcher for switching between Info
and Source tabs. Beyond giving a newer look and feel to the demo, this also
provides an example for how to use a GtkStack.