gtk3 missing types

This isn't really a new bug, as many of the past dozen or so hav dealt with the
same topic. But it's more convenient to have a tracking place for this
specific list.

A large number of types in gtk are paired, there's Foo and FooClass, and
beginning with gtk3, there may also be a FooPrivate. Where they are paired,
the actual type structure is defined in _Foo and _FooClass.

These 32 types are flagged by building against Gtk headers as not actually
having _Foo defined. All are missing FooClass if it exists upstream, and those
that have a typedef of FooPrivate are also missing that. Searching for these
in the upstream headers shows there are more missing types with related names
as well: GtkLevelBarAccessible GtkLockButtonAccessible GtkScaleButtonAccessible
GtkSpinnerAccessible GtkSwitchAccessible.

Here's the current list. All of these types need to be enabled.

GtkAppChooserButton
GtkAppChooserDialog
GtkAppChooserWidget
GtkApplication
GtkApplicationWindow
GtkBuilder
GtkCellArea
GtkCellAreaBox
GtkCellAreaClass
GtkCellAreaContext
GtkComboBoxText
GtkCssProvider
GtkEntryBuffer
GtkGrid
GtkInfoBar
GtkLevelBar
GtkLockButton
GtkMenuButton
GtkMountOperation
GtkNumerableIcon
GtkOffscreenWindow
GtkOverlay
GtkRecentAction
GtkRequestedSize
GtkScaleButton
GtkSpinner
GtkStyleContext
GtkStyleProperties
GtkSwitch
GtkThemingEngine
GtkToolItemGroup
GtkToolPalette