After commit #a7bc5f07, these plug-ins assert in local_grad_data_refresh().
A comment is clearly stating that the widget must be allocated, so make
sure that the callback is only connected at allocation.
Mainly fixing GimpRGB comments and
parameters that are unused (or in unused
functions).
GimpCircle and GimpGradientChooser
have color conversions ported to use
GeglColor exclusively.
This name was really irking me because it's not a button (anymore? Maybe it used
to be just a button). Depending on the specific widget, it will have several
sub-widgets, including a label. And it can theoretically even be something else
than a button.
So let's just rename these widgets with the more generic "chooser" name.
