(Linux) Extend support for the XDG Base Directory

[SgAkErRu]

Problem

At the moment, it is not possible to customize the default configuration of SMPlayer using system-wide settings (in /etc/xdg directories).

Solution

Add support for XDG directories on Linux at the first launch of the program (i.e. the local config has not yet been created) and when -config-path CLI option is empty.

Related PR: #1103