gschem: Fix hotkey icon display more elegantly.


Instead of creating a whole new GtkCellRenderer subclass, we could
just connect into the "notify::stock-id" signal and rewrite the
GtkCellRendererPixbuf's properties on the fly.  This is a much
lighter-weight solution!

This reverts commit 89d51ea544420506b91c3ac3d643a5c365cb807b.
This reverts commit 25625af959da02723fc6b3800616eab6bf002401.

Affects-bug: lp-1227368
gschem: Make "Major symbol changes" list expand to fill dialog height.

Closes-bug: lp-1118216