Add a PlacesSidebar example.

* examples/book/placessidebar/examplewindow.[h|cc]:
* exampels/book/placessidebar/main.cc: New files.
* examples/Makefile.am: Mention the new example and its files.
PlacesSidebar example: Fix compiler warnings
PlacesSidebar example: Minor cleanup
PlacesSidebar example: Fix with gtkmm from master.