Improve the quick grade GUI in assignment module - popup window navigation GUI and pull-down grading menu

OK, the proposed improvement was initially caused by changes to assignment module to correct the problem caused by using the "Gradebook Plus v2" (GBPv2)..

The GBPv2 gradebook allow users to change a grade to whatever they want (no error checking). If you put in grade of 555 (out of max 100), it still takes it, and then when you go to assignment, you won't see any grade (it would show "NO GRADE").

To fix that we (SFSU) changed the grading in assignment to use text fields (originally it was pull-down menu allow from 0-100). Our instructor also provided some feedback that they LIKE the "text fields" better (for entering grades). However, because the assignment module was not originally designed for text fields, so we must now do error checking. So grading in assignments is limited to 0 - 999. If assignment uses scales, then the scales dropdown will appear.

We also changed the behavior of each of the buttons available in the popup menu. (again based on feedback from our users and support team). See attached screenshots for "BEFORE AND AFTER"

[New GUI have these buttons:]
Previous - New button that saves changes and moves to the previous student to grade
Save Changes - Used to save and close the window. Now just saves the data and refreshes the window with feedback
Cancel - Closes the window
Next - Saves changes and moves to the next student to grade

Thoughts or comments? I have also included a quick and dirty patch (originally designed for 1.8.2), which probably need some rework for moodle 1.9.x. Note that we also added a help button right next tot he navigation GUI...(not included in the uploaded diff)