renamed plugin descriptor to conform with gedit3 conventions
modifications to support gedit3
Create some test cases so it's easier to regression test changes.
Redo the algorithm again as it got confused by multiline comments with a single space for indentation.
Add another test case with mixed spaces and tabs.
Add test case that confuses the current algorithm.
A single line with the wrong indentation was enough to confuse the current
algorithm. Try to make it more robust by having it look for changes in
indentation compared to the previous line.