Refactored Session manager to singleton

[kralmichal]

Issue #84

[mluzarow]

Hello kralmichal; thank you for taking the time to contribute to this little project! There are a couple things I have issue with in this pull request. One being that your editor seems to have automatically truncated the tabbing from all lines that do not have code on them; this is a minor issue but if you could remain consistent with the existing style of the code files in the project, I, and any other repo owner that you contribute to, would be very appreciative.

Additionally, you made changes to the code style, which again goes back to my above point of not staying consistent with the existing style of the existing codebase.

[kralmichal]

So sorry I will fix the issues.

Have you considered having .editorconfig file in the root of your project? https://editorconfig.org/
It'll prevent this sort of things from happening.