redesign UI for hiding/showing classes

[masuhar]

In the current implementation, there is a feature that shows/hides specific classes. The UI of this feature is a list of class names with check boxes, which are not nice. Consider better UI and implement that.

One idea is:

• for hiding, right-click a node in the graph, select a menu item "hide".
• for showing, provide a button "Hidden classes" which shows a dialog that lists a hidden class names. The user should be able to deselect some of the hidden classes.