Add alternate view for widget list

[alejandroautalan]

Another customization is to add the possibility to choose between your view of the widget palette (an accordion of flexible containers with icons) or an accordion of listbox of icons and names together, because it cost a little to recognize the widget using only the icon. It's easier for the user to use icon and name together, because the widget icons are not standard icons (Icons that everybody understands without any explanation like "save file", "open file", "cut", "paste", ,"copy" and "find" icons). My proposal is identical to the toolbar of PAGE Tkinter builder. The listbox should be a treeview because the Tkinter listbox doesn't accept icons for the items.

refs #89