Prevent most corruption of tray icons and reduce by half incomplete rendering of them, replace a never-built code block that could not be built, and replace (deprecated) direct expose handling with gtk_widget_hide/gtk_widget_show_all

*Note that this is needed for GTK 3.23 but may help remaining icon issues with GTK 3.22 as well and is simpler code in any case

The background is in https://gitlab.gnome.org/GNOME/gtk/issues/85 . One of
the conclusions, in https://gitlab.gnome.org/GNOME/gtk/issues/85#note_264804
, is that mate-panel needs to properly transfer focus on alt-F1 keyboard
shortcut.

It used to work only by luck before, only because gtk used to
deactivate itself during a keyboard grab. But as discussed in
https://gitlab.gnome.org/GNOME/gtk/issues/85 that behavior poses
accessibility feedback issues, is not coherent, and keyboard grab
feedback will not be available in wayland anyway. Thus @ebassi saying
in https://gitlab.gnome.org/GNOME/gtk/issues/85#note_264804 that not
transferring focus properly is the actual bug.

This change explictly switches to the menu bar after saving which X Window
had the focus, and on menu bar deactivation restores focus to that X Window.

Fixes #851

This a backport of commit f0f4c5e1217eefd46edf9f98633fb32967c67461.

find . -name legal.xml -exec cp ~/Devel/mate-user-guide/mate-user-guide/C/legal.xml {} \;