OnExit/OnClipboardChange now accept a function, see details:

- Removed OnExit (command) and OnClipboardChange (label).
- Introduced OnExit() and OnClipboardChange(), the syntax is similar
  to that of OnMessage():
    OnExit/OnClipboardChange(["FuncNameOrReference"])
  The function returns the old function (if it had been set).
  Calling it with an empty string as the parameter will remove the
  handler.
- An OnExit function can receive a single (optional) argument which
  replaces A_ExitReason.
- If the OnExit function returns true, the Exit sequence is aborted,
  otherwise the script ends normally. For convenience, ExitApp retains
  its special OnExit-bypassing behaviour inside an OnExit function.
- An OnClipboardChange function can receive a single (optional)
  argument which contains the value (0/1/2) which in v1.x was stored
  in A_EventInfo.
OnExit/OnClipboardChange: fix return value if there was no previous func
OnExit/OnClipboardChange command syntax: add OutputVar for return value
Merge branch 'alpha' into v2-onexit-onclipboardchange
Minor code formatting change
Refactor result token handling and function call code, see details:

- Introduced ExprTokenType::InitResult() and ::Free()
- Introduced a new overload of Func::Call() that takes a variable
  number of arguments, and their type; for simplified building of
  the parameter array required for function calls
- Changed existing code to use the functions described above
Fixed a crash occurring when `return "string"` returns to CallFunc().
Fixed `return "just a string"` to not literally include the quote marks.
Fix FileInstall to use A_WorkingDir for non-compiled scripts
v1.1.15.03
Added capability to define property accessors in classes.
Fixed objects to allow this[prm1]:=x in __Call to affect the behaviour.
Merge pull request #27 from fincs/AutoHotkey_L
Merge remote branch 'fincs/v2-funccall-refactoring' into alpha
Revised ACT_THROW implementation.
Fixed premature freeing of result by OnExit/OnMessage/OnClipboardChange.
Revised Func-calling code.
Misc refactoring, optimization and comment revision.
Optimized returning dynamically-allocated values from functions.
Revised RegEx callouts.

Callouts are now able to throw exceptions or Exit correctly.