Bug 44504

There's some big changes here.  It's best to first look at all of the added test cases in TestFormulaParser to get an overview of the work.  (The new tests can be merged in first, to observe POI's current behaviour with

Added support for parsing of '%' in formulas.
Fixed operator precedence (and confirmed associativity):
	(), %, ^, * /, + -, &, = <> > >= < <= 
Simplified missing arg parse logic and confirmed with test
Added support for error literals (like #REF!) in formulas.
Fixed parsing of string literals
Simplified logic of rendering IF() and external functions

Started support for evaluation of external functions.

Made FormulaParser detect when the input string has not been consumed.  Removed all occurrences appending a semicolon to the formula before parsing (that code made no difference before).

Performed major refactorings inside FormulaParser to fix all of this.
Added class FormulaParser.FormulaParseException to help distinguish specific parsing errors from internal POI errors.