Add the desktop entry to /usr/share/applications upon installation on linux

Hi, firstly - I'm very impressed by the project and am taking notes from the documentation, code, design, project structure to benefit my own projects, props to the team! I already have some extension ideas and would love to contribute to the project.

A quick fix for the AUR install (would be applicable for any other linux packages) - the .desktop entry should be added to `/usr/share/applications` too, in order to appear in the desktop environment app search.