Necessary modifications for deb packaging

[hsbasu]

• Add valid category in desktop file
• install css to the python source directory
• Remove byte-compiled code pycache
• Use absolute path in code to access resources from code
• Use prefix option in the toplevel meson build file
• Put the python sources in the system's pythonpath

[soumyaDghosh]

Thanks a lot sir, for this. This will also remove the patch that I use in the snap package. BTW, you can use snaps in your distro, as we can work together on it.