Improve steam path/exe configuration

[coldfix]

Follow up on #18, #19, #23, missing features from current solution:

• document command line options in README.rst
• add support for reading settings from $XDG_CACHE_HOME/[...], yaml/json/cfg/toml/vdg?
• if neither command line options nor config file settings for steam path/exe are given, fallback to previously used settings (store in $XDG_CACHE_HOME or $XDG_DATA_HOME or $XDG_STATE_HOME?)
• where is the flatpack binary com.valvesoftware.Steam installed? If it's not on PATH, we may have to change find_exe accordingly
• generalle improve or simplify autodetection(?)
• integrate path selection into UI
• allow user to configure multiple presets (default, native, flatpack, home1, home2, ...) and add some sort of dropdown or radio button in the UI to select which one to use
• move all acolyte related files out of the steam path and into the XDG paths (config/data/cache/runtime/state)