menu, add the ctrl-s shortcut to toggle the sections

Now that we can easily create favorites, it make sense for a parent or
teacher to select only the relevant activities and hide the sections.

Wen the sections are hidden, the favorite change is disabled

Still missing:
- a way to do the same on touch screens
menu, add an option to show/hide the sections

Hidding the sections can be done through a persistant option available
in the config dialog. It can also be accessed via the ctrl-s shortcut.

This option is very useful in relation to the new favorite feature.
In normal mode you set the activity you want with the favorite. Then
you hide the sections so that the children see only the favorite. When
the section is hidden you cannot change the favorite.