added Offset method to selection class

[zWolfrost]

This PR aims to close my issue (#661).
It doesn't implement any GUI options, but hopefully it makes the job much easier for people that know how to add the buttons.
I have tried my best to respect the project coding practices but i am kinda new to doing pull requests, so i would really like if this was tested first.
To get into technical details, i have chosen the "jtMiter" JoinType as i think it's the most fitting between these options.

[cameronwhite]

Thanks for working on this!
I'll need to do some testing of it (and also think about how to expose it in the UI), but the code looks good to me. Just had one minor style suggestion noted in a comment

[cameronwhite]

The changes look good, thanks!
I tried testing this out by just hacking it into the selection tool to always expand the selection, and it behaved as expected - I just noticed that the surface parameter is unused and could be removed

[zWolfrost]

Hi, just for carification, i added the unused surface parameter because the "Invert" method of the selection object also has it unused, so i figured i should add it as well. Anyway, i'm glad it works. Do you want me to remove the parameter?

[cameronwhite]

Thanks, yes please remove that parameter (and also may as well do this for the Invert() method too)

[zWolfrost]

There you go. I didnt remove it from invert method as well because that would require to edit every call of it and remove the paramether there as well, and my pc is not available at the moment.