Various small changes + add a feature to optionally disable the previous button

Hi, I have never submitted anything before to Github.

Currently in GIT I have a number of small IzPack commits which I have submitted as separate items.

These:
- Fixed a number of small typos / spelling errors
- Delete one unused variable
- Add a feature to optionally disable the previous button being shown on a particular panel. (would need a docs page update once submitted)
- Fix panel numbering. It was always 1 number too many

I have a number of other major changes (not yet in git) which I would like to submit but would like feedback on the above changes first.

I have been using IzPack with my various changes for about a year,

NOTE: I'm completely new to GIT

James