Init'ing static wxBitmaps in the global scope happens before the GUI is
ready, which segfaults, at least on GTK+. This can happen in, for
example, the Python module. In normal use, the singletop/kicad
loader has initialised the GUI first, so it doesn't manifest there.
The introduces INDICATOR_ICON, which is a very simple class holding a
bitmap that can toggle on or off.
The ICON_PROVIDER class then provides icons to INDICATOR_ICONS, which
means the class can be used for more than just row indicators.
A default row icon provider is also provided for use in the standard row
selector.
John Beard