- move the code that sets the tool options' "defined" and "serialize"
  properties from GimpToolInto to GimpToolOptions so they are always
  set correctly.
- make GimpToolOptions fix broken serialized NULL tools automatically.
- make sure a GimpToolPreset's tool options always has a tool set. If all
  fails, set a NULL tool explicitly and let the logic in GimpToolOptions
  find the right tool.
- set GimpToolPreset's "use" booleans to FALSE for context properties
  that have no effect on the tool.
- set GimpToolPresetEditor's toggle insensitive for these properties.