Add meson build file

[gavr]

something like

project('lesson3', 'vala', 'c')
dependencies = [
    dependency('glib-2.0'),
    dependency('gobject-2.0'),
    dependency('gtk+-3.0'),
    dependency('gee-0.8'),
]
sources = files('Application.vala', 'Main.vala', 'Window.vala')
executable('app_name', sources, dependencies: dependencies)

[Alessandro]

I've added the meson file in the branch related to the lesson where I explain the meson build system.
Each branch is related to a specific lesson so users can download the proper code matching the exact video.