Add GTK4 support

[btzy]

This PR is for (experimental) GTK4 support. Resolves #24.

As this backend is experimental, GTK3 remains as the default backend. Use -DNFD_GTK_VERSION=4 to activate this new backend.

Blocking issues:

• The "Save" button in the dialog is sometimes disabled if the user does not click on an item in the file/directory list. This doesn't make sense in the context of a file save dialog though - it shouldn't be necessary to select something in the list, if the user types something into the text box. This is important but I'm not sure how to fix it - help wanted.
• SaveDialog has "pressed"/"button-press-event" signals that need to be replaced with gestures. In progress, but doesn't work yet.

Other unresolved issues (non-blocking):

• X11 pop-under issue not yet applied to GTK4.
• Downloading a non-local file might take a while, but it is impossible to cancel. On Windows, the file is downloaded while the dialog is still open (this is automatically done by the file dialog), so the user can cancel it.
• There is a warning about the lack of transient parent for all dialogs, and the dialog is not modal; this is probably unresolvable.