* README.md: meson -> meson setup
* docs/tutorial/C/index-in.docbook: Note that VolumeButton is deprecated.
* examples/book/buttons/scalebutton/examplewindow.cc: Make it even more
similar to VolumeButton.
* examples/Makefile.am:
* examples/book/meson.build: Remove book/buttons/volumebutton.
* examples/book/buttons/volumebutton/*: Remove.
* examples/book/dropdown/search_font/examplewindow.cc: Change a comment.