Refactored glLabel and glView

Refactored glLabel and glView

Refactored glLabel and glView code:

- Moved selection tracking and storage of current label defaults from
  glView to glLabel.

- Removed complexity of synchronizing lists of labels between glView and
  glLabel.

- Current defaults for new objects are now kept in glLabel, not glView.

- Now follows a more faithful MVC pattern.  As a consequence, the
  Object editor and property bar are now fully independent views of
  glLabel and do not need to be aware of glView at all.

- Refactoring reduced total SLOC count by about 1100 lines of code.

- This should potentially simplify the addition of an Undo/Redo
  capability, because selections and object lists do not need
  to be kept synchronized.