In the gradient-tool editor, when responding to an endpoint size-
entry "changed" signal, make sure there is a selected handle, and
bail otherwise. We can receive a "changed" signal without a
selected handle, if the endpoint is deselected while a change to
the size-entry's value is in progress, causing the the tool GUI to
be unmapped, and the changed value to be committed.
Rename gimpblendtool{.c,-editor.{c,h}} to
gimpgradientool{.c,-editor.{c,h}}. Note that this commit only
renames the files; the actual changes are done in the next commit,
so that git doesn't consider them new files.
