build: Various meson related improvements

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.