Port to GTK4

Closes #365 (closed)

Current status:

• All tests are passing
• The image on the About page is broken It's the same in the upstream build. Probably needs a proper .desktop file.
• Segfaults on termination are resolved
• SIP plugin account window seems to work correctly (although I don't have an actual SIP account, more testing would be appreciated)
• Made a real call, and it worked (to the extent that the hardware is supported 😅)
• Unrelated aarch64 CI failures are theoretically resolved now
• CI needs to be updated so that we can use a more recent version of libadwaita (at least 1.3) using 1.2 as of !714 (comment 2069872)
• Update to libcall-ui 0.2.0 release (blocked on World/Phosh/libcall-ui!86 (merged))
• Other CI failures should(?) be resolved by @julianfairfax's MR to align with the upstream Flatpak configuration