Fixed engine advance bug.
The engine would advance too far in situations like this: pattern: 'ab', input: 'aab'. When the regex failed at index 0 ('aa' doesn't match 'ab'), the engine would advance to 2 instead of 1.
Added 'regex index' arrow to WinForms UI.
Always shows the initial index where the regex starts an attempt at parsing. Only advances upon success of failure of the entire regex.