port to gtk4

[deltragon]

~Based on https://github.com/slgobinath/SafeEyes/pull/560, will rebase the first two commits away once that is merged.~ ~Needs https://github.com/slgobinath/SafeEyes/pull/558 for the trayicon - with the trayicon plugin disabled, this can be tested as-is.~

The changes in the .glade xml files are rather noisy, they were partially done using gtk-builder-tool. I mostly made sure that things still looked correct visually after running the tool.

Opening this as draft for now, as it relies on the two PRs above. Additionally, this is a rather big PR, and I'd like to split it into things that are backwards compatible with gtk3 (like most changes in the .glade files) and things that are not. It also likely needs some changes in the configurations/dependencies.

Fixes https://github.com/slgobinath/SafeEyes/issues/367. Fixes https://github.com/slgobinath/SafeEyes/issues/521. Fixes https://github.com/slgobinath/SafeEyes/issues/473.

[archisman-panigrahi]

@deltragon Is this ready?

[deltragon]

@archisman-panigrahi No, it needs #560 and #558 to be merged first.

[slgobinath]

Thanks @deltragon

Can someone help testing these PRs? If anyone confirms the changes, I'll merge them and release.