This adds basic meson introspection support based on the meson API
for 0.50.0 (current meson master branch).

This patch also adds a GUI for changing all build options for a
build directory.

For a new build directory:

• Use the development version of meson
• Make a new build directory and set the meson executable in the advanced options
• The default build options should load after this
• Play around with the options
• Click OK and manually check with meson configure that the build options are correct

For an existing build directory:

• Open the meson project configuration
• The current options should be complete and match the actual values from meson configure
• Changing and aplying changes should also update the meson configuration
• Nothing should segfault :)