[WIP] Improve version info

[s-martin]

• save used git commit hash to version file
• use saved git commit to determine if same version is already installed

see #828

[MiczFlor]

Hi @s-martin
the conflict reported here by github refers to the installation section for spotify. Not sure which one is the latest version. Looks like the code has moved on? Could you take a look?
I will be more present now, I promise :)

[s-martin]

I fixed the conflict.

The check, if same version is already installed is still missing, but we could add that later.