Fix the creation of TotalCross modules (tcz file) using Java headless (…

…#45)

Deploy uses TotalCross' Image class to convert unsupported image
formats to PNG, and this process relied on classes from java.awt
and Toolkit, which may not be available on headless distros.

Changed implementation of ImageLoader to perform this conversion
using ImageIO instead, with some refactoring along the way for
clarity.

Fixes #27

Co-authored-by: Fabio <flsobral@gmail.com>