Gtk4Prep: Replace dialog.run () and filechooser.run ()

[jeremypw]

The run () functions is replaced with show () together with a handler for the "response" signal as will be needed in Gtk4. The dialogs are explicitly created modal. In some cases where there is code that must wait for the response, it is necessary to use async functionality and yield as dialog.show () is not blocking.

In passing

An unused dialog is removed
show_all () is moved into the Preferences dialog class where it would normally be.

[danirabbit]

Any reason you're using show instead of present?

[jeremypw]

Valadoc says Gtk.Window.present should not be used in Gtk3 because of timestamp issues. It will be OK in Gtk4 though.

Using show () does mean modal must explicitly be set to true in order to get a modal dialog. I have now gone through and made all the custom dialogs created modal and shown in the same way.

[danirabbit]

Don't forget to update POTFILES since you removed a file

[danirabbit]

Since this is a pretty large branch now, can you break out the async function changes into a separate branch? It'd be nice to have its own commit in git history. If it's too much of a pain, don't worry about it :)

[jeremypw]

No problem, it has expanded somewhat ...