Custom UI button ordering

[BryonLewis]

resolves #491

• adds an expansion panel to the Custom UI section of the Context bar that displays all of the attributes that are visualized with buttons and allows a drag/drop reordering system for them
• Adds ordering to individual shortcuts in attribute shortcut list as well
• Updates attribute shortcut list to preview the buttons and have a heading for the rows
• Updates defaults for adding buttons (sane descriptions, default button name, remove by default witll use the mdi-delete icon and be red
• Added an icon preview to the append/prepend icon for the attribute shortcuts section
• updated Action Shortcuts to be orderable as well as updated the interface for the preview of the button