[GTK4] Add dialog_run

Add a function Inkscape::UI::dialog_run() to allow us to keep using GTK Dialogs in a synchronous way, even though the API tries to force us to use them in an asynchronous way, which would require enormous amounts of rewriting. Port all uses of GTK::Dialog::run() to this new function. Verified dialogs still work for a few common dialogs (open/save, pdf import, extensions).