Refactoring meson architecture

[tfuxu]

This PR mostly makes meson structure in this codebase more readable by adding new constants, and by moving some parts of src/meson.build to a more central meson.build.

Changes:

• Create new constants in info.py.in: rootdir and app_id,
• Remove (where possible) redundancy and switch to new constants,
• Move some parts of src/meson.build to more central meson.build,
• Add new if statement that checks if the host has a python3 binary available,
• Rename info.py.in to constants.py.in,
• In src/window.py, add new if statement that adds devel CSS class to a window if build_type is set to "debug".

[0xMRTT]

Hi @tfuxu

Thanks you 😎 for the PR. It's exactly what I wanted to do.

[0xMRTT]

@all-contributors please add @tfuxu for code

[allcontributors]

@0xMRTT

I've put up a pull request to add @tfuxu! 🎉