Allow different event sources

[PeeHaa]

It would be nice if people are able to connect different event sources to the project.

This would involve making the interfaces more generic where needed and perhaps some other changes.

• [x] Move the label texts to the backend
• [x] This needs to implement a Listener interface
• [x] Replace hardcoded namespace in the event factory

[PeeHaa]

https://github.com/ekinhbayar/gitamp/commit/3c11e1c852a4ba6179b88f09f652a3690ec51f6c