Support for user icon sheets.
If the ../datafiles/interface_icons directory exists, we will first try to load icons from it,
if an icon cannot be loaded, the built-in one will be used.
The icon names can be found/copied in the "Icon Viewer" add-on,
examples of file names: CUBE.png, SNAP_ON.png, TOOL_SETTINGS.png.
Images can be of any size, not square images will be cropped.
Some test icons: