Work Progress - Feb 2 - Feb 8

* Change the behavior when a codelet is dragged over another codelet in the scriptlet window:
1) If there is a legitimate place to put the codelet, it should highlight, but if there is none, and you release it, it hovers.
2) If it highlights, and you release it, it will attach to that highlight.
* Fix bugs on insertion position highlighting and codelet insertion (previously, if there are more than one event codelets in the scriptlet window, we may not be able to insert method codelet.)
* Fix the bug that the selection area of the event codelet grows in tandem with the invisible y-slack size (abolish yslackarea)
* Fix the bug that the selected codelet(s) disappear in the scriptlet window by a click-and-release action (w/o drag)
* Properly clear the previous insertion position highlight
* The rectangular insertion highlights for box insertion points (like the conditional block of IF) are now displayed without improper displacement.
* Show error message if the invalid property is selected.

* Highlight the problematic object in property panel?