Remove unused import.
Use a constant for the settings filename.
Disable the fight button if no valid warriors are selected.

It was possible to click the fight button even if no valid warrior were
selected. This resulted in a mere uncaught exception, and was rather
unfriendly.
It was also possible to click this if invalid or inexistant files were
selected, which could be even more confusing.
Disable the Jar selector textfield.

Typing (or pasting) a file into this field did not update the class selector,
so the general behaviour was broken. Since there's a filepicker (and it has a
textfield itself), let's rely on that.
Merge pull request #1 from hobarrera/disable-fight-button

Disable fight button