Add C++11 override markers.

Adds the markers for gui2::twidget::place.
Add C++11 override markers.

Adds the markers for gui2::twidget::find_at, for both the const and
non-const function.
Add C++11 override markers.

Adds the markers for gui2::twidget::find, for both the const and
non-const function
Add C++11 override markers.

Adds the markers for both overloads of
gui2::twidget::impl_draw_background.
Add C++11 override markers.

Adds the markers for both overloads of
gui2::twidget::impl_draw_foreground.
Add C++11 override markers.

Adds the markers for both overloads of
gui2::twidget::impl_draw_children.
Add C++11 override markers.

Adds the markers for gui2::twidget::disable_click_dismiss.
Add C++11 override markers.

Adds the markers for gui2::twidget::create_walker.
Add C++11 override markers.

Adds the markers for gui2::twidget::demand_reduce_width.
Add C++11 override markers.

Adds the markers for gui2::twidget::request_reduce_height.
Add C++11 override markers.

Adds the markers for gui2::twidget::demand_reduce_height.
Add C++11 override markers.

Adds the markers for gui2::can_wrap.
Merge branch 'override_markers'
Remove some old revision number cruft.