change default icon path, add icons

Now all default resources should be put under <egui_config_path>/, which
is ~/.egui by default, and separated into three sub-directories, namely
`img', `icons' and `doc', like the schema used by EgPro. But the old
`.icons' is changed to `icons'.

An old test program `test_icon' is adapted to the new interfaces and
added to compile system.