Get rid of GtkBin

The eventual goal is to get rid of GtkContainer. For now, lets deal with the bins. The plan is to change every widget that is currently deriving from GtkBin to derive from GtkWidget, and give it a "child" property.

• GtkWindow
• GtkAspectFrame
• GtkButton
• GtkComboBox
• GtkPopover
• GtkFlowBoxChild
• GtkFrame
• GtkListBoxRow
• GtkOverlay
• GtkRevealer
• GtkScrolledWindow
• GtkSearchBar
• GtkViewport