This fixes restoring of brush properties (size, spacing angle etc.)
from presets, which was utterly broken before. The fix consists of
two parts:

- In tool_manager_preset_changed(), always copy the brush properties
  again after setting the preview on the tool options, in order to
  override brush properites that get copied from a linked brush when
  that brush gets set on the tool options.

But no amount of copying stuff again and again would help without:

- In gimp_context_set_by_type(), don't use g_object_set() to set the
  object (brush, pattern etc.). Instead, build a GValue and call
  gimp_context_set_property(). This may seem odd, but avoids a
  g_object_freeze_notify()/thaw_notify() around the g_object_set(),
  which was causing "notify" to be emitted at the very end, after
  everything this context change has triggered. GimpContext is an
  essential core object and there is an expectation of a reasonable
  order of signal emissions and callbacks being called. The "notify"
  at the end was keeping any callbacks of the context's "foo-changed"
  signals to override anything an earlier callback had done, if a
  "notify" callback was overriding that overriding again.

This was probably the reason for a lot of odd behavior observed over
the years. In fact, I have been searching for this for at least 5
years.