and "close" functions; some used true to indicate success, others to
indicate failure.
This fixed bug [ 1792359 ] when exiting, cancel button does not behave
as expected
M src/edu/rice/cs/drjava/model/AbstractGlobalModel.java
M src/edu/rice/cs/drjava/model/SingleDisplayModel.java
M src/edu/rice/cs/drjava/ui/MainFrame.java