Improve File picker

[skidunion]

1. Ability to select multiple files
2. Remember last folder opened
3. Default to obfuscator working directory instead of JRE location

[x4e]

I fixed the working directory in a recent commit, it will feature in the next release. Other ideas are good, will see about them. I think number 1 might be a limitation of Swings file picker though.

[weisJ]

As to 1 you might want to look at JFileChooser#setMultiSelectionEnabled