Add a GimpPaintTool::is_alpha_only() virtual function, which
subclasses can override to indicate whether painting only affects
the alpha channel (assuming FALSE by default).  Override the
function in Gimp{PaintBrush,Ink,Clone}Tool, returning TRUE when the
current paint mode only affects the alpha (as per
gimp_layer_mode_is_alpha_only(); see the previous commit,) and in
GimpEraserTool, returning TRUE when the target drawable has an
alpha channel.

When the function returns TRUE, and the target drawable doesn't
have an alpha channel, or the alpha channel is locked, show a BAD
cursor modifier, and raise an appropriate warning when attempting
to paint.