Qt IconButton should let its owner be responsible for connecting signals

`traitsui.qt4.helper.IconButton` extends `QtGui.QPushButton`, and its `__init__` does two things:
(1) Configure the visual things, e.g. setting the size limit
(2) Connect a slot to its `clicked` signal.

The widget would be more reusable if it does not perform the second step, but instead let its caller (e.g. `FileEditor`) does the signal connections. This will make the signal connection/disconnection more obvious too.

It could just be me: I don't normally expect instantiating the widget should have the side-effect of connecting any signals.