Make config and log files conform to the XDG Base Directory Specification

[bastidest]

Is your feature request related to a problem? Please describe.
I noticed that a log file (~/.fotokilof.log) is created in my home directory every time I start the application. Seeing log files there is very unusual on most Linux distributions.

Describe the solution you'd like
I suggest to follow the XDG Base Directory Specification when running on Linux. This allows the user to define directories where logs, settings and data are stored. For this application this would mean:

• save log files in $XDG_STATE_HOME/fotokilof/... or $HOME/.local/state/fotokilof/... if the XDG_STATE_HOME environment variable is not defined
• save config files in $XDG_CONFIG_HOME/fotokilof/... or $HOME/.config/fotokilof/... if the XDG_CONFIG_HOME environment variable is not defined