Use path relative to executable to load shaders and icons

[niekschoemaker]

This allows Codewalker to be opened from things like start menu, and overall makes it more reliable with launching, since the shaders are always in the file of the executable.

This would also allow files to be opened by drag and dropping on codewalker, and other features down the line.

[dexyfex]

Ouch, would have been much better to create a helper function somewhere for this. Also can't be adding random backup files :P

[dexyfex]

Manually added an improved version somewhat based on this