GTK4 Port: Part 1

Qiu Wenborequested to merge
crab2313/evince:gtk4-part-1 into evince-next

I'm going to split !373 (closed) as three parts:

  1. libdocument, backend, properties, thumbnailer
  2. libview, libmisc, previewer
  3. shell

This MR is the first part. We start by disabling all build targets depend on GTK3. When a build target is considered finished, we enable it at once.

Edited by Qiu Wenbo

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