Replace custom ImageIcons with UIManager constants

[vbmacher]

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);
...