SDK-15583 - Mouse cursors disappear under certain circumstances.
* There was a race condition occuring, and a private field was being modified in an inconsistent manner
-- A menu selection callback set the variable to true, to show a custom cursor
-- Then a mouseout event fired from the app, removing all cursors, but without unsetting this variable
-- Then a mousemove callback saw the variable was set, hid the system mouse cursor, until the next setCursor()
* Now we unset that variable in a central location whenever there are no more custom cursors
* Removed stale code
QA: Very yes. Please make sure that cursor functionality hasn't changed in FP since Flex 3, and in AIR apps with context menus -- in particular, any test case that restores the custom cursor after using a context menu.