Load images from a configurable path

[jackpot51]

Loving pastel so far. It is now being packaged for Redox.

The only issue is that the icons need to be loaded from an absolute path. It would be good if the base path could be passed in the compiler environment using env!, such as ASSET_PATH

[robbycerantola]

I've made some little changes so the working path will be set at compile time based on the target os .
Now it works both in linux and redox.

[jackpot51]

Cool!