Tool icon pixbufs are baked at construction time using the button's
current style context foreground color. Without this, switching the
desktop theme (e.g. light <-> dark) at runtime leaves the editor's
icons stuck in the previous color while the rest of the UI updates.
Subscribe to Gtk.Settings notify::gtk-theme-name and
notify::gtk-application-prefer-dark-theme; on either, replace each
themed button's child Gtk.Image with a freshly-recolored one.
Disconnect both signals from PerKeyEditor.shutdown so the editor
doesn't leak handlers across openings.
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