gtk4 port proof of concept

[zipy124]

In response to a request for a gtk4 port ( #140 ) here is a proof of concept. It is messy and not designed to be merged straight away, but is functional in the core aspects. I'm just putting it here in-case anyone wants to beat me to finalizing it as I am rather busy at the moment, and most of the hard-work is done.

What is working:

• locking
• unlocking
• authentication
• password message when entered wrong

What is not working:

• caps-lock indicator
• removing/adding monitors (likely doesn't work as signal monitor-added and monitor-removed no longer exist
• finished signal doesn't exist in the new lock mechanism, should use un-locked instead

What I haven't tested

• Modules/plugins such as powerbar

Apologies for the poor code quality, this was all coded rather quickly, and I am not familiar with using gtk3 or 4.