gschem: Breakout individual notify signals for selection properties
gschem: Hook help menu items up to actions.

When the help menu functions were converted to actions, the menu
wasn't updated properly, so the help menu stopped working.
gschem: Get &hierarchy-documentation action working again.

Ensure that the SRFI-1 "any" function is available.
gschem: Add GschemCellRendererIcon class.

In gschem we'd like to be able to associate a single icon name with
each action.  If the name matches a stock icon, we'd like to show it,
but if not we'd like to fall back to looking up the icon in the
current icon theme.

Unfortunately, this behaviour isn't possible when using a
GtkCellRendererPixbuf: you have to *either* use "icon-name" *or*
"stock-id".  The GschemCellRendererIcon subclasses
GtkCellRendererPixbuf and provides the desired fallback behaviour via
its "icon-id" property.

Affects-bug: lp-1227368
gschem: Show stock icons in hotkeys dialog.

Use GschemCellRendererIcon to display icons in list of hotkeys.

Closes-bug: lp-1227368