Use fallback for symbolic icons

* examples/book/actionbar/examplewindow.cc: Don't use magic number for
icon size. Allow fallback icons when using symbolic icon names.
* examples/book/headerbar/examplewindow.cc:
* examples/book/popover/examplewindow.cc:
* examples/book/searchbar/examplewindow.cc: Allow fallback icons when using
symbolic icon names. Some icon themes lack symbolic icons. Bug #723124.