GtkColorSwatch: Avoid style context underflow

The last change accidentally removed the later restore, and
left the earlier, so we ended up with a restore-save sequence.
Thankfully, GtkStyleContext warns about this.
GtkSearchBar: don't peek out

When the searchbar is hidden, a 1-pixel-high piece of it is
still peeking out, because visible widgets are always allocated
at least 1x1. Work around this by setting child-visible to FALSE
in this situation.

https://bugzilla.gnome.org/show_bug.cgi?id=724096
GtkListBox: Support placeholders in ui files

This makes the gtk_list_box_set_placeholder functionality
available from GtkBuilder

https://bugzilla.gnome.org/show_bug.cgi?id=738111