gtkplacessidebar: add API for show drop hints

gtkplacessidebar: add API for show drop hints

It is convenient to allow applications to show all the drop
targets at once. This improves the user experience with drag
an drop.

The new API allows the application to set the gtkplacessidebar
in a mode where invalid drop targets are insensitive and it
adds a "new bookmark" row. This mode is intended to be set
when the application is aware of a dnd operation and needs to
be stopped kwhen the application is aware that dnd operation
was cancelled or ended in a different part than gtkplacesisdebar.

The context parameter is unused in this patch, but will be
used in next patches when the sidebar will use a GtkListBox.
The reason of being unused now is just convenience.

https://bugzilla.gnome.org/show_bug.cgi?id=747793