Reduce discrepancy of building tools

[hongquan]

Due to the need of publishing our app to various channels, the build method are now too diverse:

• First, we use Poetry both to manage dependency and build, with the pyproject.toml file being the main script for building. However, the 2nd purpose failed, because our app cannot be distributed via PyPI (due to the inclusion of desktop-integration files like icons, *.desktop file).
• Then, we need to build Debian package. We need to generate setup.py file from pyproject.toml, because Debian doesn't support Poetry.
• Next, we need to build Faltpak package. Because of it, meson.build is added.

So, we are having 3 build scripts: pyproject.toml, setup.py, meson.build. Pretty cluttering now.

[hongquan]

Debian team is attempting to support pyproject.toml file: https://lists.debian.org/debian-python/2020/04/msg00061.html

[hongquan]

I got rid of setup.py in favor of Meson.