Organize icons in directories

For now, moved the only activity-stop icon to 'actions', but more will
come.  It is the same organization as sugar-artwork GTK.  This is to
anticipate late reorganizations.
Add zoom icons

A few sugar icons to make web activity samples, but we'll move them
all later.
Set toolbar height

Previouly was defined by the children buttons.  But if the toolbar has
no buttons inside, it must have the same height, because it will
contain other widgets.
Style input text