meson build system has received several improvements. The list of
improvements is as follows:

- The prefix used for different variables has been changed to `gtr`.
- Indentation has been changed to that most commonly used.
- Unnecessary `meson.build` files have been removed.
- GTK documentation generation has been fixed.
- `schemadir` location is now checked from GIO's pkg-config file
  when available.
- `profile` has been removed and debug builds are considered now
  development builds.
- `documentation` option has been renamed to `gtk_doc` to be
  consistent with different GNOME packages using the same option.
- meson post install script does not build paths based on
  installation prefix because these might vary.