port to gtk4

[deltragon]

Based on #560, will rebase the first two commits away once that is merged.
Needs #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 #367.
Fixes #521.
Fixes #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.

[deltragon]

This is almost ready now.

I have tested this on multiple platforms, and found one more issue which is in #655 and can be merged before the switch to Gtk4.

Additionally, I split out #651 into a separate PR to be merged after this - that one is simply an improvement on Wayland enabled by Gtk4.

While in theory, there are changes to the glade files that should be compatible with Gtk3 as well - those changes are still very big and would need to be tested manually, so we don't really gain much by splitting them out.
In addition, the only risk would be small UI/styling differences, where the update to Gtk4 on its own is the biggest cause of change anyways.
So I think we should just merge this as one, once #655 is merged.