Replace custom ImageIcons with UIManager constants

All toolbar icons (or other standard icons) should be using constants from UIManager instead of keeping icons in resources, e.g.:
```
btnBrowse.setIcon(UIManager.getIcon("Tree.openIcon"));
```

Then, emuStudio will set these icons globally: 
```
Icon openIcon = new ImageIcon("open.gif");
UIManager.put("Tree.openIcon", openIcon);
...
```