SpinButton|GradientSelector|FontCollSel: Replace/fix GdkEvents for key input & focus

commit 6de9b77beeaf2586c82a99444782e492e9ffe851 (HEAD -> djb_SpinButton_GdkEvents)
Author: Daniel Boles <dboles.src+inkscape@gmail.com>
Date:   Thu Aug 31 11:10:03 2023 +0100

    FontCollSelector: rm GdkEvent, fix delete & insert
    
    * Replace ::key-press-event with EventControllerKey
    * Fix delete key insisted frame label be "Collections", BUT:
      - that does not account for translation
      - the untranslated label set elsewhere is "Font Collections"
      - this is the only instance of the widget, so why check at all?
      - why would other instances not be allowed to delete, anyway?
    * Fix adding new could make on_selection_changed() throw a CRITICAL, as
      sometimes the iterator does not have a parent. If so, just bail early.
    * Replace raw `new` with Gtk::make_managed()

commit edee3513d23510f7374487bc740d80ef72fe2091
Author: Daniel Boles <dboles.src+inkscape@gmail.com>
Date:   Wed Aug 30 17:30:06 2023 +0100

    FontVariants: Cleanup/remove commented ::key-press

commit 5d07749fb6827b3f6e07bf63fb11be2f927be035
Author: Daniel Boles <dboles.src+inkscape@gmail.com>
Date:   Wed Aug 30 17:25:30 2023 +0100

    GradientSelector: GdkEvent → GtkEventControllerKey

commit 974949560042ba4c3ce9124cb0fb649511fd01bf
Author: Daniel Boles <dboles.src+inkscape@gmail.com>
Date:   Wed Aug 30 17:14:36 2023 +0100

    SpinButton: Replace GdkEvent for key input & focus
    
    I almost gave up and became a mountain / log cabin person because
    nothing made sense, until I realised that, apparently per the Standard,
    constructors inherited from a base class will NOT call a user-provided
    default constructor in the current class. Absolutely crazy, isn't it?
    So, we can't inherit ctors, but must instead forward via our OWN one.
    I generally love C++ and try to evangelise it, but sometimes it's tough.